In the episode where Betty and Wilma are taking judo lessons, the first time the instructor comes over, he has a moustache. The second time we see him, he does not. The third time, he doesn't, either, but when he starts to bow to Wilma and Betty a couple of times, you can see his moustache flash on and off many times. It's really obvious, since the entire shot is focused on him. I guess the animator couldn't make up his mind!