Ok. Two comments/suggestions.
1. If you are going to play an (already memorised) piece from the score in concert, do yourself a big favour, and practice it with the score. Retain the memorisation by all means, but having run through it repeatedly from the score means you are far less likely to get lost in the event of accidents.
I strongly agree with you here!
andrew wrote:2. I strongly believe that if you have a memory lapse, don't go back. Go forward. Mentally select a nearby bar which constitutes something structurally significant (this shouldn't be too difficult if you have thought about the architecture of the piece and if you keep calm). Find a way to connect, harmonically or melodically, even rhythmically, wherever you currently are at on the piano to that bar, join them up and continue. It looks much more convincing to an audience than stopping and starting. Half of the time they won't even know! (Disclaimer: I'm good at improvisation, so the process seems easier to me than maybe it actually is).
Good tip. Next time I have to perform, I'll practice improvising in random places - if that's even possible. I'm not a good improviser. I'm also afraid I would randomly start playing "Misty" because that's what I always play when I'm just screwing around.