Joke from Scoop (2006)
 

Woody Allen's character walks into a music room and picks up a french horn.
He says "This reminds me of an old dirty joke about the french horn player who could only sleep one way with his wife".

It took me a second but a day later I still find it very funny.

The movie has a few good lines :-)

:confused: i dont get it...

Yeah. I've told this joke to a few musicians and they didn't get it either. :-(

A french horn is played with one hand in the bell of the horn (to correct tuning).
Guess it's not common knowledge.