It happens to me a lot, but I’m always encouraged by the fact that I’m able to do everything the way it should be when I play immediately before and after my lesson. I’m always afraid that I’ve let my teacher down, since she sometimes asked if I’ve had enough time to practice during the week. I have noticed that there are pieces that should be simple but for some reason take a long time, three or four lessons at least, before I finish them.
As far as brain mush, I sometimes can’t remember which colored string is F and which is C.
All in all, the best remedy for me is to go back and play something I learned a month or so ago. Very reassuring.