Other mistake: When the 4 humans rush out of the pool house, there are 3 aliens not dressed in their human skins watching them leave. The problem is: only one alien has eyes. We've seen the aliens out of their skins heaps of times and they always have eyes.
Other mistake: After the Griswolds leave the German couple's house, the man says "Who the hell were they?", and the woman says "Beats the shit out of me." However, since they slept there overnight, they had enough time to discuss it in private much earlier.
Other mistake: A long-nailed woman types the opening credits in a computer, but in a couple of instances (the Playboy Playmates, for example) you can see her type away when the credits page is already complete and the names are already there. (00:01:00)