This did not happen to me or anyone else I know but as I was playing this morning on a court next to a private lesson I thought about this. Let's say I was in the middle of a match, say at the tail end of the 2nd set with an opponent and there is a private lesson next to me. My opponent goes to pick up our balls after a point but grabs a ball that was hit over from the lesson next to us. The hypothetical ball could be completely dead as a lot of lesson balls are and could seriously affect play, especially on the clay surface. If my opponent (either intentionally or unintentionally) grabs the wrong ball, serves with it, can I call a let after the point is over? I don't think you can really get a good look at the ball during a point to realize it's not the same ones you've been playing with. Also, obviously there is no way for me to realize it's not the right ball before the point starts. Just curious, thanks!