Five minutes before the big concert starts, the conductor sees his clarinetist with lots and lots of clarinet pieces, hectically trying to reassemble the instrument.
The conductor asks: do you think it is a good moment for taking the instrument apart? Only five minutes to go!

The clarinetist looks up and says: "I didn't take it apart, it was the viola player!"

Conductor goes to the viola player: "Why did you do that?"

Viola player: "For revenge! The clarinetist put one of my viola strings out of tune!"

Conductor: "But that is no reason to take the clarinet apart completely!"

Viola player: "I only did it when the clarinetist didn't tell me WHICH string ..."

;-)