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.
Sink or Swim - S6-E8
Other mistake: When Carl is trying to teach Urkel how to swim in the pool, the pool signs say 4ft and 6ft. But if you notice, when they stand up, the water is only at their wasteline. Obviously, Carl and Urkel are not 8ft tall or anything. So they are not standing in actual 6ft of water.
We're Going to Disney World (1) - S6-E22
Continuity mistake: In an earlier episode, Steve told Carl that he has to take charter flights because he was banned from all commercial airlines. Yet, in this episode, he takes a commercial airline to Florida.
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.