Re: the belief that you should know absolutely that you can play the song/piece before you perform it.

There's the oft-stated (possibly apocryphal) tale of "scat" singing being invented by a jazz musician who started to sing a song and realizing halfway through that he didn't know the words. Whether it's true or not, it does demonstrate something - that is, sometimes inspired music comes out as a result of only half-remembering the tune or the words and creating something new out of the resulting improvisation. If we want to be note or word perfect all the time, then we risk having to rely always on the printed or recorded source. Look at Miles Davis' seminal 'Kind of Blue' album - yes, they were all master musicians, but Miles deliberately kept them away from any arrangements before they went into the studio, where he gave them a rough outline format and let the music develop organically. Their not knowing exactly where the music was going was what made it so fresh.

Besides, I believe well run folk clubs are capable of structuring an evening so that the experienced performers open and close the sets, while those still 'cutting their teeth' are allowed a chance to perform somewhere in the middle.