Other mistake: Eddie drives the car through the front door. This shouldn't be possible. Whenever the front of the house is shown, we see that the porch is several feet above the ground and the only way to get to it are steps that aren't wide enough apart for a car to drive between them.
Deliberate mistake: When everyone is doing the Urkel Dance on the rooftop, Steve is instructing everyone on how the dance goes since they have never done it before, but there are sections of the dance where he is not telling them what to do, but it is still perfectly choreographed.
Visible crew/equipment: When Urkel throws the baseball at the lamp, the shadow of the boom mic can be seen moving from right to left on the wall that the closet door is on, and then moves back in the opposite direction until it is pulled out of view as Carl screams "What happened!?" (00:22:22)