Source: somewhere on the 'net.
Note: You won't get it unless you understand music theory.

Three notes walk into a bar, a G, an Eb, and a C.

The bartender looks up and says "We don't serve minors."

So the Eb leaves and the other two have a fifth between them.

After a few drinks, the G was out flat, and the whole experience was diminished.

Eventually, the C sobers up, sees one of his friends missing, the other one passed out, and a policeman standing over him. And to his horror he realizes that he's under a rest.