everyone has off days, in whatever field they pursue. all of the great musicians had at least one concert where they didn't perform up to their usual level (although I'd love to have even a 10th of the ability of Richter on his worst day!).
for me, the key is to recognize that you're having an off day, and to not try to push through it but just let things play out, relax, and then start over the next day. often times I even find that an off day is a prelude to a period of rapid learning and progress. if, however, I try to force myself to perform at the usual level, it only leads to frustration and further degradation in my (already limited) abilities.
I had a day a few weeks ago where nothing seemed to work right surgically. surgery has a rhythm, just like music, but nothing was flowing, none of the tools seemed to work right, and the operations were just sloppy. don't worry too much, no harm came to anyone, but it was still frustrating that things that should have been easy were anything but. but the next day, I was operating as well as I ever have, and for the life of me I can't figure out what the difference was.
brogers, you're definitely right about scientists feeling like failures. I think that it's common in any relatively solitary, introspective pursuit. I know I sure feel like a failure when it comes to research, but I'm pretty sure that I'm not just making this up in my head--my publication record is terrible!
anyway becky, as far as piano-specific advice goes, during a bad day I do try to play just a little, but I stop when I feel myself getting frustrated and I don't really work too hard on challenging, new material, but instead back off and play a few easy, fun pieces, just enough to keep the fingers going. I'm a little different though--piano is a hobby, and a luxury at that as I have so little free time in my life, so I especially can't afford to be angry when I'm at the piano--the time is far better spent on any number of other things.
Topic: Down in the dumps. coping.