Revealing mistake: Carl, Eddie, and Steve are out ice fishing. At one point Carl falls through the "ice" and you can clearly see that the "ice" is made of some sort of styrofoam. It looks like the stuff used for packaging. The snow looks just as fake.
Revealing mistake: In the episode where the Winlows' try that shampoo and their hair falls out, you can see they are wearing bald skinhead caps, especially Laura.
Revealing mistake: In the episode where Steve is helping Richie with a science project, Steve turns on the tornado machine which malfunctions and causes a very powerful wind to blow through that begins to decimate the kitchen. While this is happening, Richie is forced through a door and out of the kitchen by the wind. When Richie is again, as he is holding onto the doorway, support wires are attached to him lifting him up.
Revealing mistake: When Carl, Steve and Mr. Looney first get locked in the bomb shelter and the door slams shut you can see it bounce back slightly. It doesn't stick closed.
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.