I've watched Martin Carthy dry up on stage in the middle of a song... he had to finish the story in precis and without accompaniament.

I read somewhere, years ago, can't remember where... that the brain remembers stuff when it's in the same condition as when it first learned it. Hence, there are some tunes and songs I can only remember when on the receiving end of 4 pints of beer. As I no longer drink that much in one session, I can't remember a lot of things! If you learn something and then recreate the conditions in which you learned them, you should be able to recall them.

Explains a lot about pub singers.

LTS