Sorry to bring up an old topic, but what if the competitor knows that he twisted a corner in the middle of the last F2L pair and he knows which corner he twisted. It happens that it would have been an OLL skip. If he twists it back, he is definitely not getting any advantage in solving the puzzle. It would only be an advantage if he didn't know what corner he twisted.
Also, is a judge really expected to be able to follow the way the person is solving the cube and whether it advances the solve? As Dene pointed out earlier, she would have no clue if a person was getting an advantage in a method other than CFOP. I believe one of the main points of the WCA judging system is that anyone can judge. people who don't solve the cube don't have a clue what's going on. This would mean that people who have a non-cuber judge could twist a corner to solve OLL and the judge wouldn't know.