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.
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."
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.