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.
Answer: It was just cancelled. Read more here: http://www.kellie.de/end.htm.
Myridon