"re Classical Performers - I've never known why soloists have the music there as they know it inside out anyway."

Knowing the music "inside out" does not necessarily mean that even the best of them won't have a memory lapse from time to time. Even the recitalist, standing in the curve of a grand piano, has his or her accompanist a few feet away with the music in front of them to prompt them if necessary should they falter. In full-blown opera productions, Broadway musicals, and stage plays, there is a prompter's box under the stage and out of sight of the audience where someone with a score or a script hunkers down, ready to feed a line to a singer or actor when and if necessary.

So no matter how well you--or a very highly paid, world renown singer--know the material, neither you--nor they--are immune to the occasional brain-fart.

Don Firth