Is that the nature of the horn?

In one orchestra, the first horn player was out with a cold. So the trombone player took over the first horn's solo. Yet, when the trombone player played with the orchestra, the conductor had a troubled look upon his face.

The conductor said, "Well, it's okay, but it doesn't sound like the horn."

The trombone player replied: "Do you mean the timbre?"

Conductor: "No. The timbre is fine."

Trombone player: "What is it then?"

Conductor: "Play slightly out of tune, like a horn!"

This is a famous joke. This shows how often horns are out of tune and how hard they are to play well.