I recently had to withdraw from a performance as I wasn't given enough time to prepare adequately for it. As Krummhorn said, to the casual listener it might have sounded OK, or even great - I knew it wouldn't have been what I could deliver as my best so decided it was better not to to it at all.

I get nervous enough playing for concerts - not so much for a regular Sunday service anymore, only for the bigger ones, but that's because I know in my heart I know what I am playing and I can play it well (self praise is no praise!, I know that!!!!)

I do get nervous when introducing a new voluntary - and 2012 my target is at least another 5 voluntaries, after a horrendous 2011, I need to get back to playing to the standard I know I can.
2012 = nervous year then :-)