Other than in a rehearsal or a recording, I can't see any justification for having crib sheets. If you don't know it, don't sing it. Performing a song is a method of communication. It's on a par with acting. Who would expect actors to read from scripts?

Mistakes are acceptable in either context. A great part of stage craft is in dealing with the mistakes and esuring that the communication continues.

Whenever I perform a new song I expect to forget some of it so I work out a stratgey to deal with it when I do. That takes the pressure away and makes it less likely that I will forget some of it.

Communication is everything. You can't communicate what you don't know.