They Stole Our Act

It's the big night for a given musical group, usually a show choir. Everyone in the group is feeling great, and ready to perform a routine or set that they've practiced to perfection, only to find out that they're going last, and the performers set to go before them are performing the same routine. The group now faces the problem of figuring out how to make their performance original in the precious little time that they have before their turn on stage.

Examples of They Stole Our Act include:

Film

The choir of Sister Act 2 finds that they're sharing "Joyful, Joyful" with the choir that has gone before them.
Bring It On. A high school cheerleading team is humiliated at Regionals when the team performing before them uses their routine. Turns out that the guy who taught them the routine had been peddling it to many different schools.
Happens in A Mighty Wind. Throughout the film we see The Folksmen practicing a song called "Never Did No Wanderin'," but on the day of the concert, the first group to go on, The New Mainstreet Singers, perform it, forcing The Folksmen to pick a different song to open with.

Live Action TV

In one episode of Jeeves and Wooster, Jeeves sabotages Tuppy Glossop's act by arranging it so that all the performers before him sing "Sonny Boy", the song he plans on singing.
Glee: New Directions faces this problem during sectionals, when their set list is leaked to the other school's glee clubs.
- Happens again, when Will and Bryan Ryan realize they're doing the same song for a performance of Les Misérables. The solution? They are forced to sing it as a duet.
Emmmett Otter's Jug Band Christmas: The eponymous jug band discovers that a guy going before them in the talent show sings the old favorite "Barbeque", which they had planned to sing. Fortunately they have time to go out in the alley and work up a new number.
In the episode of The Wonder Years where Kevin learns piano, this happens to him at the end-of-year recital.
Slight variation of this done on the TV show version of Ten Things I Hate About You. Bianca and Chastity find out ahead of time that they are doing the same song, and Bianca has to do a song switch. After Chastity's Auto-Tune breaks during the show, she and Bianca agree to sing the original song together.
This happened in an episode of Drake and Josh. After a chorus stole and performed Drake's song, Drake and Josh instead perform Soul Man from The Blues Brothers.

Manga and Anime

Skip Beat!: Moko and Kyoko have worked out a great routine for their audition, and... The Libby has stolen it from them. They change the skit on the fly and pass the audition.

Western Animation

The Simpsons: In the tryouts of a kid Idol show, Lisa's original song was 'Hush Little Baby', but the girl ahead of her sung it really well. Homer makes up a song about Springfield on the spot for her and she gets into the show.
Foster's Home for Imaginary Friends: Bloo plans a hiccup act for a talent show, but his rival Blake Superior uses the same act, only with burping. This leads to a burp-hiccup competition between the two that Bloo wins. Neither of them, however, win the talent show; they are beaten by a friend who plays his armpits.

Other

There's an old joke that uses this trope. Two novice preachers will be giving sermons sequentially at a gathering. The one who'll speak first overhears the other preacher practicing. Realizing the second preacher's sermon is superior to his own, he steals it. The first preacher delivers the (stolen) sermon to thunderous applause. The second preacher starts with, "That was the finest sermon I have ever heard. Since I cannot improve upon it, I will instead repeat it word for word." He does so and wins undying fame.