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.
Dream Date - S7-E22
Other mistake: Steve and Laura kiss at the prom, and the moment is never mentioned again. In fact, during the Season Nine episode "Drinking and Jiving", Steve is surprised when Laura kisses him passionately, while she is drunk. He is surprised enough to mention it the following morning and offer to start a relationship (then Laura pretends that she doesn't remember).
Continuity mistake: Carl tells the establishment's owner that Eddie and Waldo are under the age of twenty-one. However, in the episode "Best Years of Our Lives" (also Season 7), Waldo tells Maxine that he worked on a parade float every year in high school, which makes seven floats. If he failed three grades and started college in the previous season (with Eddie), he would be at least twenty-one years old by now - probably twenty-two.
My Broken-Hearted Valentine - S3-E18
Other mistake: When Steve is wearing his Cupid costume and waiting for Laura in the Winslows' living room, the doorbell rings. He is obviously excited and ready to pose before seeing Daniel there. This does not make sense because Laura would not ring the doorbell to enter her own home. In fact, she does not ring it when she does arrive shortly after Daniel.
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: 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.
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.
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.
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.
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.
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)
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.
It Didn't Happen One Night - S5-E2
Other mistake: When the folding bed contracts after Laura turns the lights off, it's very obvious the two legs sticking out from the bed are fake or that of a mannequin.
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.
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)
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.