Not really a mistake...well, maybe an editing mistake. I was watching Last Crusade a few nights ago and noticed something I had never seen before. Had to check it out to make sure I didn't imagine it.
When Elsa hands Donovan the false grail, there's a shot where he holds it up and says "...this truly is the cup of the king of kings". In the background of that shot you see Elsa turn to Indy and move her head side to side as if to say "no, it's not". The very next shot is her turning her head (again) and giving him "the look" with no head movement.
I dig that I can still see new stuff in movies that I love.