I had a kind of off experience today too. Things were going well - I was playing Rachmaninoff with a cellist. Big audience. All was under control. It was gorgeous. I had my beta blockers and was determined to focus and go through this smoothly. The notes were correct. Then.......I must have slowed down during a solo part, and when the cellist came in, he decided to pick the tempo back up. A little abruptly, I thought. I wasn't even aware I had slowed that much, and his tempo seemed crazy to me. Suddenly, I didn't know where he was, and for 3 long measures I stuttered and panicked and just couldn't believe this was happening - it had never happened before. Things got back under control, but I was kind of in a different world after that.
As to your question, my experience is that often after a difficult part, our brain kind of takes a vacation and that's when we screw up.
For me, a BIG challenge is focus. This piece I have been working on for a long time, so it is nearly, but not at all completely memorized. The hard part is remembering to constantly look at the measure ahead and focus on what I'm about to do. To constantly do this without letting my mind wander is hard. I'd love to see a map of what my mind does during a performance.
I imagine it's like this:
OK, build to here, left hand quiet, look ahead, now shift to these notes, look ahead, go to here, listen to cello, this is great, I love this music, they must think this is beautiful, what does my piano teacher think of this, look ahead, shift to these notes, leap to here, now pp, now build, why is the cellist speeding up, sit up straight, I wonder if I wore the right outfit, look ahead, here comes this part, keep it FF, now drop, look ahead, voice, DARN I forgot to look ahead, oh shoot did I play that note right? keep going, OK, watch out! big part! big big big part!! oh no, disaster! Mayday! Help! I hate performing! Lets just try to salvage this thing. OK, look ahead, deep breath, at least finish it on the right notes, don't make a fool of yourself. OK, finally, it's done. Now bow without rolling your eyes and grimacing, bow again because they are clapping so hard. Now when people tell you it was wonderful say thank you, don't mention the screw ups. Oh darn, I mentioned the screw up.
And isn't it all fun.
Pizno