Continuity mistake: From the outside of the house, you can see the staircase and door are to the far left of the house. But from inside the house, you have a fireplace and the staircase to the right of the door. So you know it's not the same house they use in the show.
Visible crew/equipment: In the episode where Carl is "fishy sitting" for his boss, Steve is explaining to the boss how he swallowed the fish. You can clearly see the studio lights reflected in Steve's glasses.
Continuity mistake: Whenever the characters in this sitcom fall through the roof, they end up on the living room floor. The house is a very conventional design with a second floor and an attic, so they should end up in the attic or on the second floor.
Plot hole: The history on Urkel's Isetta is all wrong; in one episode he mentions that he paid $4 for a sunroof that he claims he bought used, however his uncle paid him $50 to take it away. In another, he claims his car has a speedometer that goes up to 30, but before it had a chalkboard. In "Hot Rods to Heck," the speedometer goes to 60 mph. The car would also screech, putter, sputter, smoke, break down, etc. but it seemed to be just fine in "Driving Carl Crazy," ''It's Beginning to Look a Lot like Urkel," and "Hot Rods to Heck."
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.
Continuity mistake: The Winslows' front porch as seen from inside their house features a lattice wall with vines and furniture. However, establishing shots of the house show no such features.
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.