Portal Slam

A character tries to enter a magic portal, often at high speed, only to discover that it is no longer there. Maybe the magic has been sealed or the portal cannot be opened in the presence of Muggles, i.e. the character can't show it to his family or friends. Maybe the portal is an Empathic Weapon that lets you through only when it's in the mood.

Whichever reason, the Portal Slam is normally a sign that the fantasy world the portal gave access to is irrevocably inaccessible. This is a common method employed at the end of a fantasy/science fiction series to bring things back down to normal. If the character manages to get back to the world by alternate means, he'll usually discover that the world has drastically changed for the worse with black magic brewing.

Less commonly, the Portal Slam is used near the beginning of the story, as the protagonists figure out how to work the portal by trial and error.

See also Teleporter Accident, Tele Frag, and Portal Cut.

Film

 * The child Boo in Monsters, Inc.. finds only an ordinary closet instead of a portal to Monstropolis after Sully and Mike are obligated to let the closet door be destroyed. Earlier, the same thing happens when Sully and Mike are banished, finding a big metal door in the middle of nothing.
 * In the film version of The Hitchhiker's Guide to the Galaxy, Arthur is afraid to go into the portal at Magrathea after the others. By the time he gets up the nerve and runs at it full speed, the portal is inactive and he ends up sliding through it.
 * The Phantom Tollbooth (the movie, not the book) eats itself after Milo goes through it.
 * The Matrix Reloaded features a literal backdoor in the programming, whereby the right key in a lock will allow access to anywhere. If the door is closed then the portal shuts and the door goes back to leading wherever it did before. Neo tried to shoulder charge one, only to punch straight through and end up outside the building.
 * One of the the Twins goes to gruesome extents to prevent this from happening. He stuck his arm through the door to keep the heroes from shutting it, and stayed like that even after they unloaded a pistol into his arm. Luckily for him, his wounds heal completely when going into ghost form.
 * A famous scene from Austin Powers: The Spy Who Shagged Me has Dr. Evil running into his unactivated portal-style time machine, only to smack into it.
 * In the Napoleon scene in Time Bandits, The Bandits escape through a timehole, and just as the angry soldiers reach it, the timehole collapses and all the soldiers crash into each other and fall down.
 * In The Adjustment Bureau, the Adjusters have the power to use any door as a portal. Similar to The Matrix, the portal disappears when the door is closed.
 * In the original Stargate movie, Jack O'Neil, Daniel Jackson, and their team go through the gate to Abydos with the expectation they'll be able to go back to Earth right away. Only problem is, Daniel was expecting the gate address for Earth to be written down somewhere in the same room as the gate.

Literature

 * Harry and Ron rebound painfully from the sealed gateway to Platform 9-and-Three-Quarters in Harry Potter.
 * In The Phantom Tollbooth, Milo finds the morning after that the tollbooth has vanished, with a note in its place that explains that the tollbooth will find its way to the next needed child.
 * The doorway in the wardrobe into Narnia vanishes when Lucy tries to show it to her siblings the first time. A portal to Narnia only seems to appear when you're not looking for it, according to Professor Kirke -- and indeed, in CS Lewis's other Narnia books, every portal only works once, as opposed to the three times the wardrobe does.
 * Used again in The Magician's Nephew, with an actual splash; Digory and Polly jump in a pool that they know is a magic portal and only get their feet wet. In their case, though, the portal still worked; they just weren't using their magic rings correctly (as they quickly figured out).
 * Princess Quinn of Dian Curtis Regan's novel Princess Nevermore attempts to return to her homeworld by leaping back into the pond she emerged from. All she gets is soaking wet. The spell must be reactivated  before she can return.
 * A much different version occurs in Neal Asher's Gridlinked. Because the planets that the gates are on are moving at different velocities (and directions), there is technology in place to keep someone from stepping through one end of a portal and flying out the other side at thousands of miles an hour. The sabotage of this safety feature has disastrous consequences and starts off the plot.
 * In Dean Koontz's Lightning, any  attempting to make a "jaunt" that would result in   is violently "bounced back" to the point from which the jaunt was originally attempted.
 * Variation from In the Keep of Time: since it isn't possible to "splat" against a portal which consists of simply opening a real door, the children are instead kept from traveling through time by the need to warn the men of the English ambush, then by the key's disappearance. Although the magic of the key comes and goes, this turns out to be a Red Herring and not a Chekhov's Gun, as it seems that once "unlocking" a time period, the key remains ready to use until the bearer returns to their own time. Something of a They Wasted a Perfectly Good Plot moment, although subverting reader expectations here was also clever.
 * Cowl forces this on Harry and Lara in the Dresden Files, just as they were about to escape the C4-rigged cave.

Live Action TV

 * The TV series Goodnight Sweetheart ended with Gary's Portal to the Past closing forever - leaving him in Post War Britain.
 * In the Doctor Who episode "Father's Day", disruptions in time caused the interior of the TARDIS (which is, of course, bigger on the inside than on the outside) to vanish, leaving only the police box shell. No, don't ask how this makes sense. At least it makes more sense than the original idea; it was supposed to FALL APART.
 * It didn't vanish. The TARDIS interior exists in another dimension. Merely the portal between dimensions vanished, presumably due to the disruptions. Falling apart would make exactly no sense, because the interior and exterior aren't actually related.
 * It also makes more sense than a similar scene in "The Time Meddler"; when the Meddling Monk's TARDIS had its dimensional controls removed, the console room shrunk to fit the exterior shell!
 * The Episode "Girl in the Fireplace" gives more examples of this, with time portals between a futuristic spaceship and 17th century France. One portal the doctor uses is attached to a mirror, which breaks on the way through and won't let him back in. The title fireplace is another (malfunctioning) portal, but while time passes normally on one side years pass between each trip on the other, with more than one serious plot consequence.
 * Though not quite the same, the title portal of Stargate SG-1 has an item known as an "iris" that can be closed over the portal, which prevents most physical matter from fully manifesting itself on Earth's side of the wormhole if the bad guys decide to use it in an invasion attempt. Coming through the Stargate after the iris has been closed results in what is known as "bugs on a windshield death". Presumably they have technicians just to wipe the back side of the iris after this happens.
 * Captain Carter says "It doesn't even allow matter to reintegrate", so there shouldn't be any residue. This doesn't explain why there's an audible "THUMP" and the iris visibly pulses each time an object splats against the iris, though.
 * Stuff still comes through as subatomic particles, it just doesn't have enough room to reassemble itself into molecules or Jaffa. The "THUMP" is presumably the iris being bombarded with the stream of low energy protons and electrons which used to be the traveller.
 * Atoms are reintegrated. This is used to provide allies without an IDC with a primitive one-time method of contacting SGC: Throwing a box containing iridium through the gate and allowing traces of the element to be detected on the iris.
 * The THUMP is just for your sick, twisted viewing entertainment.
 * It could also be seen as the Rule of Perception in effect.
 * A non-lethal and variant occurs when the gate deactivates moments before a traveler reaches it (hopefully before partially entering it and suffering a Portal Cut). For example, in Michael, Ronon pursues Kenmore who has taken Teyla hostage, tries to leap through but just lands painfully behind the inactive gate.
 * Vortices disappearing when people try to jump through them happens an awful lot in Sliders. If it's a good guy, they'll always find another vortex. If it's a bad guy, they're usually trapped until the good guys can deal with them. Rickman, the Big Bad of season three, meets his end this way when a portal is just over a cliff. When it shuts down just as he's leaping for it, the Portal Slam is soon followed by a standard slam.
 * A Portal Splat was used to remove Mr. Marshall from Land of the Lost when the actor playing him wouldn't come back for the third season. Fiddling with the controls of a pylon, he opened a portal and slipped through, but a tremor knocked the control-pedestal over, causing the portal to vanish before Will and Holly could follow him.
 * The brilliant cartoon episode of Farscape has quite a few of these, with painted-on wormholes. Used purely for comedy, without any of the dramatic implications.

Tabletop Games

 * In the Dungeons and Dragons Planescape setting, this is a frequently mentioned hazard of plane-walking. Many portals sometimes work only under extremely specific circumstances ("Thou must be accompanied by a flame-haired tiefling whilst holding a lily and only on the third day of the second week of the month on a year with a solar eclipse on Aber-Toril"), and portals can be destroyed or closed by various beings, so it's recommended players provide their own means of transportation.

Toys

 * In Bionicle, the transportation system of the city of Metru Nui isn't made up of highways or railroads or anything like that - it's made up of chutes. Some daredevils, instead of just hopping on at the stations, prefer to jump in through the energy fields that hold the chutes in place. Time the chute-diving when the field fluctuates and you pass right through; time it wrong and the impact could be lethal (time it in-between, as Matau did in Legends of Metru Nui, and the "slam" is merely a pratfall).

Video Games

 * Semi-example: The Portal used to travel between Ages in the Myst games are the Linking Books - written on the right paper with the right ink in the right language, and have a moving image on the title page that takes you to the Age the book describes when touched. The title world of Riven collapses at the end as a result of your method of escaping. In the next game, Exile, you run across the old Linking Book for it - the panel is black, and ripples electrically while never actually opening.
 * In Riven, The Fissure was . However, in and of itself it was a Portal, as the linking book that initiated the Myst franchise falls into the player's hands The book then falls through the fissure and onto the surface of  where the player found it. Later, in Myst: Uru, you also find the telescope that fell into the fissure, also on the surface. So the player comes into the story due to  pulling a Portal Slam on.
 * More conventionally,.
 * Can happen in Halo if you place a vehicle over the receiving end of the teleporter. You can also stand on it, which temporarily blocks them, and then you move and kill them easily. It'll kill you if you stand on it long enough, but at that point you kinda deserve it.
 * Invoked in Wing Commander, where the Kilrathi find a way to obscure an intergalactic jump point, leaving the Terrans stranded and in an ambush.
 * Will almost certainly happen at least once during a play through of Portal, since you're going to screw up and place a blue portal when you meant to place a yellow one, or vice versa. Happens even more often during co-op play in the sequel once you realize that it's almost as much fun to torture your teammate as it is to solve the puzzles.

Webcomics
""Well, I guess I spend the rest of my life pretending that didn't happen or knowing that everyone I love suspects I'm crazy. This'll be a fun 70 years.""
 * In Bob and George, George just quite doesn't make it to a portal that Bob just got hit into, believing it to lead back to his own world, where George is the only person who can keep him in check. However, the Author had redirected the portal, meaning it wasn't as bad as George had feared.
 * Parodied in this XKCD strip:


 * This Wapsi Square strip demonstrates that this trope can also occur if someone simply pulls the portal away at the last second.

Western Animation

 * Wile E. Coyote's experience probably qualifies. He's repeatedly painted a tunnel on the side of a wall, seen the Road Runner go through the tunnel, and then followed it to run smack into the wall.
 * Usually accompanied by a literal 'splat' sound.
 * Sometimes, it's a paper wall set up at the edge of a cliff, and he bursts through the paper.
 * Sometimes, after the Road Runner goes into the painting, a truck comes back out of the painting to run Wile E. over. Then He gets better and tries it himself.
 * A hilarious inversion in one episode: the Road Runner goes through a painting in the middle of the road, and when Wile E. tries to follow he ends up inside the painting - where he falls down the cliff that he painted.
 * No less than three incarnations of Teenage Mutant Ninja Turtles have told the story of an artist based on Jack Kirby, whose drawings created a parallel universe that he and one of the Turtles must save. The portal stays open long enough for the Turtle to return home... but not Kirby.
 * The 1987 Teenage Mutant Ninja Turtles animated series did this frequently, to the point where it's lampshaded after the Big Bad and his mooks escape through a portal that closes before the good guys can follow: "How do they get away EVERY time?"
 * The first episode of the second season had the most famous instance of a Portal Splat. When Krang agrees to send Shredder back to Earth, the portal opens and he dashes through; but when Rocksteady and Bebop attempt to follow, the portal closes on them, and the two of them rebound off the surface of the portal mechanism itself.
 * Hsi Wu, the Sky Demon, in Jackie Chan Adventures attempted to exit the Demon Netherworld using Shendu's portal after Jade goes through it, disregarding the rule that only one person can use a portal when it opens. Sure enough, the portal closes and Hsi Wu slams into a floating rock formation that happened to be behind the location where the portal manifested.
 * In one episode of The Grim Adventures of Billy and Mandy, Billy's kiddie pool becomes a portal to another dimension. Whenever Sperg tries to use it, he just hits the bottom of the pool.

Real Life

 * People who try to enter Platform 9 3/4 of King's Cross Station to get to Hogwarts by walking through the concrete wall tend to discover this. (Side note: the wall in question is not even between platforms nine and ten, but a dummy archway off on a side wall closer to the entrance. The masonry between platforms nine and ten has been removed; the barrier in the film was actually between platforms five and six.)