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)
Visible crew/equipment: When Eddie drives his car through the front door, when they show the hole he made with the car, look at the very top of the screen, the sky stops and you can see a good two-three feet of sound stage.
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.
Answer: It is said that as Steve Urkel became a more popular character on the show, Judy became less important to the storylines, plus either her or her parents were demanding more money, causing her to be removed from the show entirely.