How would you find a missing number? I saw a woman under hypnosis suppress her knowledge of the number four. She counted "1, 2, 3, 5..." and therefore saw a total of eleven fingers. She said there were three wheels on a car. She couldn't even recognise the word "four". So the question is: how would you discover that this has been done to you? Mismatched counts such as the eleven fingers should be a clue, and when simple sums don't make any sense, you should start to wonder. But it's such a bizarre effect that you might never reach the conclusion that something so basic as a number has been erased from your brain.
Mokalus of Borg
PS - I think I would assume some kind of optical illusion.
PPS - Or else some other trick.