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.
Deliberate mistake: Richie was an infant in season one, and beginning with this season, he has been aged up to at least 3 years old while none of the other characters have aged up.
Marriage 101 - S2-E3
Continuity mistake: Steve buys the ring for Laura at the end of their class project, then sells it to Carl as a gift for Harriet. Then when he proposes to Laura he says it's the same ring from that day.
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.
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.
Other mistake: When Eddie and Carl are out ice fishing, you can tell the background is just a wall. If you look towards the bottom, you can see the line all the way across that would make it look like a wall of scenery.
Other mistake: Even though this is the first time that the kids are being introduced to the "Urkel Dance," Maxine is mouthing the words to the song as she dances.
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.
Other mistake: In this episode, they have a leak in the roof. Yet, the house is a two story house with an attic. So it should not be able to leak into the living room. Also, when Carl and Eddie go up to fix the roof, Eddie falls through the roof and ends up on the living room couch. Again, two story house with attic. Should not happen.
Character mistake: When Estelle gives her guest speech about the Tuskegee airmen she shows a slideshow. There is a photo of an airplane that she incorrectly labels as "a single engine P-47." This is not correct. The plane in the photo is a P-39 Airacobra. (00:19:45)
Food, Lies and Videotape - S3-E17
Continuity mistake: Towards the beginning, Carl enters with a new video camera and we see a regular view of the scene mixed with POV shots from the camera as Carl narrates. When he's standing at the door with his bag, in the POV shots from the camera you can see that the door is open, but in the wide shots, it is 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.