Proscenium Reveal

The device which makes it clear that the scene we're watching is a part of a show within the show or simulation. Examples could be a director calling "Cut!"; a line flub or dialogue prompt; the sudden appearance of a camera or sound crew, or a pan, zoom or cut that reveals that the action is occurring in a theatre, on a soundstage, in a classroom or on location.

A Proscenium Reveal may end a Danger Room Cold Open, a Fake Action Prologue or the Unwinnable Training Simulation. In a Music Video, it can indicate that we've been On a Soundstage All Along.

When done accidentally In-Universe, it often means the hero just ruined the shot.

Not to be confused with Breaking the Fourth Wall, in which the characters acknowledge their fictional status and/or the existence of the audience (i.e., you).

Note: Proscenium reveals can be Mind Screws for the audience, especially if they occur late in the proceedings. (David Lynch, we're looking at you.) When citing such cases, consider tagging for spoilers.

Film — Live Action

 * The dismantling of Buster Keaton's "bedroom" in "The Playhouse".
 * In the original Fame, one of the main characters is monologuing about his mother. It looks like an interview, until he flubs a line and we realize it's an audition.
 * Zig-Zagged in the climax of Blazing Saddles, where the action is somehow "real" even though it's shown to be happening on a Hollywood soundstage — and eventually most of the backlot — during the climactic Battle Royale With Cheese.
 * The cartoon short starring Roger Rabbit and Baby Herman at the start of Who Framed Roger Rabbit? is cut short by a human director calling "Cut!" This is followed by shots establishing that the Animated Actors are working on a live-action soundstage, thus setting up the concept of humans and toons living in the same world.
 * The entrance of Admiral Kirk in Star Trek II: The Wrath of Khan ends the Kobayashi Maru.
 * Meryl Streep flubbing a line during The Oner that opens Postcards From The Edge.
 * The cry of "That's a wrap!" and the applause after  in the David Lynch film Inland Empire.
 * In the Club Silencio sequence of David Lynch's Mulholland Drive, Rebekah Del Rio during her performance of "Llorando" yet, which causes Betty and Rita (and the viewers) to realize.
 * X-Men: The Last Stand opens with an action-packed scene in burning city ruins, but it is revealed to be just a Danger Room simulation after the Fastball Special.
 * The ending of Murder 101 is this. It turns the thing from merely being a bad movie into a bad movie about a bad movie.
 * Ararat does this fairly often. It lets the audience watch the movie-within-a-movie just long enough to momentarily forget that's what they're watching, until the camera pans to show the director in his chair, or someone walks on the set to correct one of the actors.
 * Moulin Rouge begins with the Twentieth Century Fox logo shown on a stage (with the Fanfare performed by the pit orchestra below), and ends with the credits scrolling by on that same stage.
 * Sucker Punch, like Moulin Rouge, opens in a theater that becomes the movie (only it never returns). And also has a weird example: Baby Doll getting lobotomized... followed by a play in which Sweet Pea in a blonde wig complains about it ending with the heroine ending that way.
 * At the beginning of Team America: World Police, a very crude puppet is jerked around in front of a laughably-bad background. This was done by the creators to troll their investors, as the film is itself done with puppets; after a few seconds (long enough that, the story goes, one of the investors yelled, "My God, they fucked us!"), the camera pulls back to reveal that the crude puppet is part of a puppet show on the streets of Paris, being worked by a much better puppet.
 * Scream 4 begins with a typical Slasher Movie opening, with two teenage girls called by a mysterious and threatening stranger... until we see the Stab title screen and realize that it was actually just an opening to the movie within a movie. Then we see two more teenage girls watching that movie, and it turns out that this is actually the opening to the sequel to that movie... yeah. It's that kind of movie.

Animated Films

 * Bolt opens with an extended action sequence in which Penny and Bolt battle a horde of mooks. After Bolt vanquishes the last of them with his Super Bark, Penny picks up Bolt and walks away—to a trailer with Bolt's name on the door. As they step inside, a bell rings, and the film crew wander into shot and start striking the set, while the "dead" bodies get up and walk off.
 * The instructor critiquing a monster's performance in the training simulation that opens Monsters, Inc..
 * Gundam 00 a Wakening of The Trailblazer starts off with a Show Within a Show retelling of the TV series' events; however, anyone at least passingly familiar with the show should instantly know something is up.

Literature

 * War Game by Anthony Price opens with a battle in the English Civil War, which goes on for a couple of pages before one of the dead bodies leans over to make a snarky comment to his neighbour, and it turns out to be a modern-day re-enactment.
 * Cat-a-lyst by Alan Dean Foster opens with a battle in the American Civil War, before the protagonist flubs his line and it's revealed to be a film shoot.

Live Action TV

 * The Supernatural episode "Hollywood Babylon" opens with two terrified 20-somethings, Wendy and Brody, in the woods. Brody runs away; Wendy calls for her friends, hears a noise, turns toward the camera and screams — unconvincingly, at a tennis ball stuck on top of a movie camera. "Cut!" calls the director. "Wendy" is actually Tara Benchley, the lead actress of Hell Hazers 2.
 * Another Supernatural example, the episode "The French Mistake"..
 * Worth noting, the episode's title is a Shout-Out to Blazing Saddles (mentioned above), "The French Mistake" being the musical number that gets interrupted by the climactic fight scene.
 * On more than one occasion in Quantum Leap, Sam leapt into a strange situation, only to discover that he was an actor in a play/on a soundstage.
 * This happens Once an Episode in F/X The Series.
 * Nikki's first flashback on Lost features a proscenium reveal. Nikki is shown pole dancing in a club, then having a confrontation with her boss. The boss shoots her, and the director yells, "Cut!", revealing that Nikki an actress working on a show about strippers who fight crime. The original plan was to have the entire episode revolve around this Show Within a Show, with the proscenium reveal coming at the end. This plan was scrapped when Nikki and Paulo proved wildly unpopular.
 * Without a Trace: Security guard sees suspicious car. He looks inside and sees a bomb, which explodes, setting him on fire. Turns out this security guard is actually a stuntman, participating and shooting of B-Movie. Then he walks away from the set and never comes back.
 * The Six Feet Under episode "In the Game" opens with a young girl home alone when a slasher-killer breaks into her house. The girl screams...then cut to inside a movie theater where we realize it's only from a horror movie that is playing.
 * One episode of Castle opens with this. A couple is having an affair when the women's husband returns. The man goes to hide in a the closet and a dead body falls out prompting a camera pan to show the shocked cast and crew of the soap opera that is being filmed.
 * The Twilight Zone had an episode, "A World of Difference", in which the Proscenium Reveal came as a surprise to the protagonist—he'd been going about his life, then suddenly found himself on a set, 'playing' himself. The episode dealt with his attempts to first understand what had happened to him, then to get back to a life he found preferable to the one he'd been thrust into.
 * Star Trek: Enterprise: "Computer, end program."

Theater

 * In Kiss Me Kate, just before the overture reaches its final chords, the conductor cuts it off and asks, "Is that all right, Mr. Graham?" Fred enters and says, "Yes, the cut's good, leave it in."
 * Noises Off opens with a housekeeper walking on stage and nattering into the phone. Then as she's walking off, she says, "I take the sardines... I leave the sardines..." and an off-stage director's voice says, "You take the sardines, and you leave the newspaper." This reveals that what you're actually watching is a rehearsal of a Play Within a Play.
 * City of Angels opens in the Show Within a Show, and gets most of the way through the expository scene before revealing Stine at his typewriter pressing the backspace key, causing the action to rewind.

Video Games

 * In Tiny Toon Adventures: Buster Busts Loose, the game is mostly set as a movie with the characters following a script, and after beating the final level, a Star Wars parody, the characters are seen carrying the props away in the background.
 * The Book Ends of Super Mario Bros 3.
 * And Mario & Luigi: Superstar Saga, shown in-game as a movie everyone was watching.