Here's one for trombonists:
Q: How many trombonists does it take to change a lightbulb?
A: Two. One holds it up and the other drinks until the room spins.
Q: How do you know when the singer's at your door?
A: Can't find the key and doesn't know when to come in.
Also, through high school, the orchestra conductor would ask for the foggotti, and we clarinetists would be immature enough to giggle a little bit. Band conductors knew better, I think.