Script Swap

Swapping sheet music is a common variant.

"Okay. The game is Cue Card Rummy. I deal."

—Garfield, preparing to invoke this trope, Garfield and Friends, "Health Feud"

A Script Swap occurs when either by accident or by external mischief, someone winds up reading or following a different document than what they're supposed to, and does so without at any point noticing the mixup.

Setting one of these up is a common Adventure Game-exclusive Stock Puzzle. The structure is simple: you've got a character in the middle of reading a speech/playing a song/giving someone instructions. Your life would be a whole lot easier if that speech/song/set of instructions was different. So you inform the character of the three-headed monkey sneaking up behind them, then swap their speech/lyrics sheet/instructions booklet with another one you acquired by irrelevant means.

A common trick to pull on the Cartoon Conductor. Whether or not this is Truth in Television is a question someone more psychologically educated than myself will have to answer.

Not to be confused with a Recycled Script.

Examples of Script Swap include:

Anime and Manga

Variant: In the manga Cat Street, Keito's script is wrongly transcribed for her by rival auditionees in order to improve their own chances of being selected. Keito cannot read kanji and asked them for help, resulting in a completely nonsensical monologue.

Comics

A slightly inventive version is performed with two typewriters in a Spy vs. Spy comic.

Film

In A Night at the Opera, Chico and Harpo swap the sheet music of an entire orchestra so that they all start playing "Take Me Out to the Ball Game" in the middle of the opera.
Anchorman: The Legend of Ron Burgundy: When Veronica switches the TelePrompTer to read "go fuck yourself San Diego", Ron reads it without missing a beat.
In The Seduction, Morgan Fairchild plays a news anchorwoman named Jamie who is being stalked. At one point the stalker gets into the newsroom and replaces her TelePrompTer feed. She starts to read it in her bland newswoman's voice until she realizes what she's saying.

"Jamie, I'm watching you..."breaks down on live TV He's gonna kill me... Please help me... Please!

In Scary Movie 3, there's a scene in which two characters type conflicting information into a teleprompter, followed by a technician typing in rude words just for laughs. The newscaster reads it all (including gibberish such as the output from just sitting on the keyboard) with a completely straight face.

Live Action TV

On The West Wing, President Bartlett is forced to nominate complete non-entity "Bingo Bob" as a replacement V.P. to avoid a long confirmation battle, and his scriptwriters complain bitterly, going so far as to pen a resounding denunciation of this ridiculous lobbyist lapdog. Inevitably, it ends up on the president's TelePrompTer by mistake. In a partial aversion, Bartlett merely gives a surprised blink and then improvises a glowing eulogy, because he is just that awesome. (And the V.P. actually requests a copy, saying he needs to be aware of what people are thinking of him.)
In Seinfeld, when Jerry and George got their mentoring notes mixed up, Bania ends up doing a brilliant stand-up routine about corporate risk management, and George gives a lecture to his colleagues about Ovaltine.
A classic accidental example from Friends: Rachel is making English trifle for dessert, but two pages on the recipe book stick together, so she ends up making a mincemeat trifle that Tastes Like Feet.
In one episode of Star Trek: The Next Generation, a computer glitch replaces the text of a play with some of Data's poetry. Fortunately, they notice this during rehearsal.
Done in an episode of Murder, She Wrote with an aging actor whose memory is so bad he has to rely on the teleprompter. While this looks like an Engineered Public Confession, it is actually a ploy on Jessica Fletcher's part to trick the real killer into exposing themselves.
In the first episode of Just Shoot Me, Maya is fired from her job in a newscast for changing the teleprompter on an obnoxious anchor, making her say that she got her frontal lobe removed and that she wet herself.
- Another episode has Nina and Maya nominated for fashion magazine awards. Maya's nomination was for an article that was changed without her permision, so she planned a speech rejecting the award and badmouthing the article. Unfortunately, Nina took Maya's acceptance speech by mistake, thus making Maya look bad when she won her award immediately afterwards.

Music

In the video for Lindsey Stirling's performance of the Mission Impossible theme with The Piano Guys, Stirling does a "Mission Impossible" Cable Drop to change the sheet music being played by a pianist.

Theater

Victor Borge, Danish comedian, conductor and pianist, would switch around his own sheet music during his comedy act.

Video Games

Telltale Games loves this trope.
- Episode 102 of Sam and Max Freelance Police does this when swapping a set of game show question cards with an easier set of questions. Possibly justified in that since Hugh Bliss isn't the normal host, he probably has no idea what the questions are supposed to be.
  - Episode 104 of the same series does the same thing with a series of cue cards for Abraham Lincoln's statements on the issues. Abe must really not be paying attention, since the poster and the sign used for the swaps look nothing like the normal cue cards in terms of font and layout... maybe this one relies on Rule of Funny.
  - Episode 205 has a grocery list swapped with a list of swear words. To be fair, both of them are written on official stationery.
- The third game of Strong Bads Cool Game for Attractive People has this done with a series of records for the Two-O Duo's dance routines, including introducing two new ones, combining this trope with Programming Game. Coach Z is smart enough not to dance off the stage if that's what the music suggests, but he's not smart enough not to air-punch his partner square in the face.
- This is done on two levels in the second Back to The Future game, where Marty has to swap the sheet music at the piano so Cue Ball plays just the right music to change the mood so a drunken officer will get in the mood to talk to him. Then when he needs a more inspirational song than the club has to offer, Marty gets the lyrics to such a song, swaps them for Trixie's lyrics to her own song, then changes the piano music so Trixie's song is played. Possibly justified in the latter case in that the two songs have really similar lyrics, and Trixie explicitly hasn't memorized hers.
The image is from the first Spy Fox game, where the player has to do this to get tango music playing so Russian Blue will dance.

Web Comics

Imps do this to Garland early in 8-Bit Theater. Garland does notice that the speech written for him by Princess Sara has been replaced by an insult letter, but that still means he has to think up something quickly, resulting in his memetic line.
- And the speech is switched back when Garland tries to present it as evidence that it has been tampered with.
Bug Martini with... credit card chip readers.

Web Original

In the Homestar Runner toon "love poems", Homestar produces an acrostic poem that spells out the name of his girlfriend Marzipan. The poem consists entirely of food items.

Strong Bad: That's not a love poem! That is a lavishly produced grocery list!
Homestar: No, here is my grocery list! "Amazing thing, amazing thing, amazing thing, amazing thing."

Western Animation

This is a favorite trick of Garfield in his animated TV series. He's done it with the cue cards for an exercise show (page quote), a disc jockey's advertisement, and a list of pet tricks for a cat show.
In an episode of The Simpsons, Bart replaces the hymn sheets and music at the local church with those of "In-A-Gadda-Da-Vida" (a Mondegreen of "In the Garden of Eden") by Iron Butterfly.
In Tiny Toon Adventures, Buster switches the lyrics on a bossy kid's performer so that his song about a monkey who looks both ways to cross the street ends with the monkey being run over.
In the Tom and Jerry short "Carmen Get It!", Jerry pulls a variation of this trick on Tom by having a sizable number of ants rearrange themselves on the conductor's lectern.
Roger would often do this to Doug such as the time where it led Doug to report that Mr. Bone wore pink underwear. Doug never caught on because he's crazy.
In the Chilly Willy cartoon "The Legend of Rockabye Point", a bear tries to keep an Angry Guard Dog asleep by singing "Rock-a-bye Baby". At one point he's playing it on a clarinet, and Chilly switches the sheet music to a loud circus theme to wake the dog again.

Other

Austrian comedy group "Die Hektiker" had a series of sketches about this trope, where a group of criminals swapped out the prepared speeches of high officials. The Austrian defense minister's speech before the NATO becomes the lyrics of "Jingle Bells", the US president's German speech before his Austrian guests a series of insults, and the Pope's German mass becomes the parable of "Jesus and the Self-Cleaning Oven".