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 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.