- Play poorly: This one is for your teacher. Play the right notes and rhythms, but do the opposite of what the music needs.
Play Chopin like Mozart, and Rock of Ages like John Coltrane. Add a swing to the sonata, or play the recitative in time.
- Start the wrong song: Get the orchestra you're in to play the wrong movement when the conductor starts them, or to play a piece that just isn't in the program.
Don't do this one on your own, because it will seem that you're trying to cover up a genuine mistake.
- Transpose: Surprise your singer by transposing the song a third up if he's a tenor, or a third down if he's a bass.
Play in a different key than your instrumentalist and insist that you are absolutely in the right key.
- Make an important change: If you're in charge, tell your band that you're changing half the program for this weekend's concert.
Hand your student an important exam they didn't know about. Or tell your cast that you decided to do the show in another language.
- Quit: Officially announce that you are tired of being a musician, and that you are quitting. And that you're selling your instrument. And your scores.
Picture from http://www.telegraph.co.uk/technology/3356868/Top-10-April-Fools-Day-links.html