Ninja Prop

This is where Medium Awareness meets Chekhov's Gun.

There are certain things we're used to seeing in certain media: speech bubbles and captions in comics, wires that make people "fly" in shows, and stage hands that move props and sets around in theater. This last one is where the trope gets its name. The classic outfit associated with Ninja (black, tight suit and a mask with a slit for the eyes) actually comes from stage hands in Japanese theater. They wore black so that the audience knew to ignore them. Imagine the shock of the audience, then, when the non-entity setting the castle walls in place for scene 4 suddenly pulls out a katana and kills one of the characters.

And thus is named the Ninja Prop. You didn't see it coming, because you were actively ignoring it as just a necessary part of the medium.

Compare Chekhov's Gun. Sometimes invokes Medium Awareness. Someone Elses Problem may be an in-universe related trope. See also Metafictional Device. In videogames, Interface Screw may qualify as a Ninja Prop, especially when the game simulates effects from external sources/other programs. In Comics, Frame Break is the most common form of this trope.

Anime & Manga

 * In one episode of Slayers, Lina sweatdrops, then grabs the sweatdrop and hits Gourry over the head with it.
 * In an early Dragon Ball chapter, Goku hits Yamcha so hard he flies into the top frame of the page and cracks it.
 * The sun and moon from Soul Eater being oddly shaped and having faces initially seems like stylization, but in the manga,

Comic Books

 * Used with some frequency in Suske en Wiske: the characters aren't above using panel lines and speech bubbles to improvise an attack strategy.
 * The characters in Asterix sometimes use the comic's panel dividers to support themselves. Also, there are two albums featuring a similar gag, where the bard is sweeping musical notes with a broom as if they were dust.

Films -- Live-Action

 * In Loaded Weapon 1, during the climax, Colt sneaks into the villain warehouse and two German, Nazi-esque guards exchange small talk, complete with subtitles. However, after they leave the scene, the subtitles remain, and Colt trips on them like they're part of the scenery.
 * In Spaceballs, after having performed an Indy Hat Roll, the heroes find themselves surrounded by Spaceball guards. Then an officer comes to gloat, but he realizes with dismay that the guards have instead captured their stunt doubles.
 * While the comic it was adapted from depicted it as a genuine plot device from the start, in Scott Pilgrim vs. the World Scott appears at first to have picked up the One Up life as a throw-away joke before talking to Ramona but

Live-Action TV

 * Monty Python's Flying Circus
 * Episode 14 has a scene where a woman is being interviewed about a gangster. Of course, like in many Monty Python skits the woman is played by a man in drag. So the audience would just think of this as the case here... right up until he says "... and what's more, he knew how to treat a female impersonator."
 * Raymond Luxury-Yacht (pronounced "Throatwobbler Mangrove") appears in two Python sketches, played by Graham Chapman wearing a very large false nose. In both sketches, Raymond treats it as his real nose, only for the other character to pull it off and point out how ridiculous it is.
 * In the Stargate SG-1 episode "200", there is a segment re-imagining the pilot as a marionette show, in the style of Team America: World Police. Everything is going fine until the team go through the Stargate for the first time... at which point the wormhole closing severs their marionette strings, and they fall in a heap on the other side. It should be noted that the strings originally weren't too noticeable on screen, and had to be digitally enhanced to be more visible, in order for this gag to work.
 * Wizards of Waverly Place: Justin is sitting with his girlfriend in front of fake French scenery. Because this is what one would expect as French scenery in a low-budget show such as this, this seems completely natural... until it is revealed that this actually is a fake French scenery in-story that Justin set up in the sub shop.

Newspaper Comics

 * A The Wizard of Id strip has a guard in a tower with a Z over his head. The invaders report that the guard is asleep, and go to attack. Cut to the tower, where the guard is holding up a fake speech bubble with a Z on it.
 * Peanuts
 * A cartoon in Here Comes Snoopy has Snoopy sleeping in a sitting position, complete with Z balloon. As his head started slumping, the Z balloon got tilted on its side.
 * In a cartoon from We Love You, Snoopy, after Schroeder finishes playing the piano and walks off, Snoopy decides to play it himself. Instead of musical notes, the staff overhead is filled with paw prints.
 * There's also a strip where Snoopy goes to sleep over musical notes that Schroeder plays from his piano. Schroeder makes him fall off by hitting a sharp note.
 * Given that Medium Awareness and constant violation of the fourth wall make up a good chunk of the strip's humor, it should be no surprise that Pearls Before Swine does this regularly.

Puppet Shows

 * In one of the Swedish Chef sketches on The Muppet Show, the Chef is trying to get his chicken to lay an egg. After it looks like she has, he angrily declares that the object is not an egg but a ping-pong ball. The humor is, of course, that the audience would expect the ball to double for an egg in the sketch, making it surprising when the Chef refers to what it really is.

Theater

 * The narrator in Into the Woods. Counts as a prop because.
 * In The Pirates of Penzance, there is a part where the Pirate King engages in a sword fight with the baton-wielding conductor of the orchestra. This was originally improvised by John Clark, the actor who originated the role on Broadway. Sullivan, who personally conducted the orchestra during the opening performance, so enjoyed hamming it up a little that he convinced Gilbert to rewrite the script to include it.
 * The Mystery of Irma Vep does this with its use of Loads and Loads of Roles, as two actors portray a total of seven characters in the play.

Video Games

 * Infamously used in I Wanna Be the Guy when obvious parts of the background,, directly attack the Kid. As does a save point.
 * Similarly, Final Fantasy VIII has a horribly surreal moment after the characters leave reality: the first thing you see when you get back to your feet is a save point, which multiplies as you step on it. Doesn't do anything else, it's just Mind Screw.
 * Whereas in Chrono Trigger, one of the enemies you can face in Magus castle are fake save points.
 * At one point in the ancient past you can try to sneak past a couple of monsters as long as you don't make any noise. The chime from touching the save point will wake them up.
 * This also happens occasionally in Final Fantasy XII.
 * One of the first things revealed about Marvel vs. Capcom 3 was that Deadpool has a special attack where he grabs his life bar and pummels his opponent with it.
 * Also appears in Syonbon Action. The clouds come to life and kill you if you happen to jump into them.
 * In MUGEN, Light Yagami reads his opponent's life bar to see what their name is so he can write it in his Death Note.
 * Metal Gear Solid: During the fight with Psycho Mantis, he uses the controller against the player. You've got to switch it to the second controller slot on the PSX.
 * Batman: Arkham Asylum features a vicious Interface Screw where the Scarecrow's Fear Gas causes the game to apparently crash and restart, complete with graphics glitches that make the player worry the game's burned out his graphics card.

Visual Novels

 * Ever 17:

Web Comics

 * The Order of the Stick loves this trope.
 * The best example would probably be the diamond from the cast page, which Haley Starshine stole from herself in order to pay for a spell in the main comic.
 * In one strip, a mute Haley holds a mental argument with herself while on the road, drowning out Elan's Blah Blah Blah dialogue. In the final panel, it's revealed that he's literally been saying "blah blah blah" the entire time, hoping to set a new world record in consecutive use of the word.
 * In the comic book compendium of this webcomic, the party uses the narrator to distract the monster guarding the entrance to the dungeon.
 * Lien knew that Qarr was up to no good from his sinister-looking Speech Bubbles (red text on a black background).
 * It's possible to feign death by drawing X marks over one's eyes and lying very still.
 * MS Paint Adventures has had a lot of fun with this, especially in Problem Sleuth. Ace Dick beating an NPC to death with part of the user interface and Problem Sleuth attacking the final boss' healthbars directly both come to mind.
 * To a lesser degree, it also happens in Homestuck. Every character has a different strange Inventory Management Puzzle called a sylladex; many shenanigans are had early on by people trying to get things out of their sylladex that they accidentally buried. But then we get to Gamzee, who, rather than bothering with doing things the hard way, reaches up into the corner of the screen where the sylladex cards are displayed and just grabs it.
 * Nemen Yi from Keychain of Creation uses the page dividers (gutters) as a throwing weapon here.
 * Cyanide and Happiness uses this frequently.
 * One of the competitors in the Coliseum Original Character Battle Tournament had this as his gimmick; he could hop between panels to make quick escapes, and use speech bubbles as shields or weapons. Unfortunately, none of his opponents were Medium Aware enough to twig to what he was actually doing or take him on using his own tricks.
 * A Loonatics Tale: Flint is somewhat medium-aware and likes to panel-hop in order to get an edge, especially when hunting; in metanarratives outside the comic proper Flint is full-on medium-aware, genre-savvy, and wash-and-wax.
 * At the beginning of Trigger Star, a jumping mook gets (gruesomely) impaled on his own speed-lines.

Web Original

 * Homestar Runner has a Strong Bad Email entitled "Virus", in which a virus infects the entire website, resulting in Reality Warping. At one point Strong Bad is able to run right out of the flash video file, and into the black webpage background beyond (and the entire video moves when he tries to jump back in). Homestar also manages to pick up the text links beneath the video. In fact, both the background and links are part of the video (and in the case of the links, perfectly functional), and the video itself is larger than usual, to encompass the added area. But since these elements look exactly as they normally do, the effect is quite surprising.

Western Animation
"Homer: I don't remember saying that."
 * The Simpsons. Homer is looking at a family portrait where Bart holds up an "I Stink" dialogue balloon behind him: