Door Jam

Perhaps the hero has just gone through the portal when it suddenly breaks, leaving him with no way back. Perhaps everyone is fleeing but the hero, probably because he is More Hero Than Thou, insisted on everyone else going ahead of him and thus didn't make it in time. Either way, he's trapped with the big boss or in some other dangerous situation, and there's no way for anyone else to come help.

Usually this is caused by a Phlebotinum Breakdown at the worst possible moment, but occasionally the situation in question could be designed to only let one person through. The plot does not usually revolve around finding a way back; it's usually a simple excuse to isolate the hero with the villain or monster.

Compare Point of No Return, a specifically video game variant.

Also, not to be confused with a Doorstopper.

Anime & Manga

 * Mahou Sensei Negima: Negi's party gets trapped in the Magical World when the Portals

Film

 * In Star Trek V the Final Frontier, the transporter has just enough power to beam up Spock and McCoy, leaving Kirk stranded on the planet with the vengeful god.
 * In The Matrix, Agent Smith shoots the phone just after Trinity gets out, leaving Neo with no exit and no choice but to fight him.
 * In the first Terminator movie, Reese explains that the time machine got blown up just after he followed the Terminator through, so "It's just him and me." The subsequent movies also featured just two Terminators going back in time, so apparently he was misinformed.
 * Or made subtle changes to the timeline that resulted in a slightly different future, just like what is stated to have happened to varying degrees in the next two movies.
 * Han Solo riding out to save Luke in The Empire Strikes Back, even though he knows that he won't be able to get back before the doors close.
 * in the new Star Trek.
 * The mirror entrance to Dracula's castle in Van Helsing.
 * In Serenity River does this on purpose to protect the rest of the crew. The door mysteriously unjams at the end, however...
 * Although it was only jammed from the one side in the first place.

Literature

 * Harry Potter and the Chamber of Secrets had this occur when an attempt to sabotage a rescue of Ginny Weasley caused a cave in to separate Harry and Ron, forcing Harry to proceed alone.
 * Harry Potter and the Philosopher's Stone has Harry and Hermione reach a wall of flame and only have one potion that will take them through the flames; thus, Harry has to confront Voldemort alone.

Live-Action TV

 * The Star Trek the Original Series TOS episode "Arena", where aliens force Kirk to duel the lizard creature alone on the planet's surface. This is a case where the hero being stranded was actually the plan.
 * Variations on this occur a number of times, notably in "The Savage Curtain" and "Spectre of the Gun".
 * Also in TOS, in "The Tholian Web" Kirk gets stranded in "interspace" with only limited air in his spacesuit, while complications ensue for the rest of the crew.
 * In the Doctor Who episode "Dalek", Rose is not quite fast enough to make it through the security doors and ends up trapped with the Dalek.
 * In the episode "The Girl in the Fireplace", the Doctor broke through a time window to the eighteenth century, breaking the connection of all portals leading to his TARDIS but one which was disconnected at the time.
 * In The Sarah Jane Adventures, when Sarah, Jo, the Shansheeth, and Colonel Karim are trapped in the room with the memory weave self-destructing and the doors deadlocked.
 * There was an episode of Jackie Chan Adventures where a portal between the demon and human worlds could specifically only let one being through. Jade was trapped in the demon world and managed to escape because all the demons were fighting over who should get to go through.
 * In Goodnight, Sweetheart, Gary Sparrow has a personal time portal that normally allows him (and only him) to travel between present-day London and London during the Blitz. But in various episodes, the portal doesn't work or it works by different rules.

Video Games

 * Done with Ramza in Final Fantasy Tactics, for one-on-one battles. The entire party is isolated in the end as well.
 * This happens towards the end of Quake IV - Matthew Kane is the only person able to go through the Strogg teleporter to face the final boss, as he has been partially "Stroggified" himself and the teleporter is fatal to humans (as one unlucky soldier discovered the hard way). His squad mates are still able to communicate with him, though.

Real Life

 * Cortez set his ships on fire after landing in Mexico, thereby making the conquest of Aztec empire somewhat of an only way to go.