I may have posted this before. A friend of mine was scheduled to sing an inspirational song to a group of boy scout -- something like "the wind beneath your wings," or similar.

He walked out and sang the first four words and then totally lost the words and the tune. He stood there for a moment with a "sheep eating grin" (or is it a "shitish grin") on his face, shrugged and walked off the stage never to be seen again on the program.