I know this is supposed to be about musical instruments but the best I can offer is experiences with my bodhran. Apart from tuning issues, there are some days when that link between my brain and my tipper runs smoothly and I can do all those cool tricks I've learned from other players without a bit of difficulty, and there are days when I can't get fancy for fear of losing the beat. In those cases, I'll play a simple rhythm w/o putting in too many variations in. As a rhythm player, it's more important to be consistant than dazzling.
Now, speaking of real instruments, I read in some liner notes about what Paul McGrattan refers to as a flautist "losing the blow". He says "You wake up one day and you can't get a note out of it!" Now THAT sounds scary!

Slan,
Rich