I once sat on stage with another performer who did stuff right off my album!
Another time, I sat back stage and crossed off each piece as other performers did them.Sure I was irritated, but none of the stuff was legally mine, and I have hundreds of things to choose from.

If I only had a handful of things to do at a given performance and someone nicked one or more of them, I'd be pissed off, but it would be MY problem, not theirs.