I'm not really sure, but I believe in one episode, one of the Bundy's (Kelly, I believe) accidently knocked the middle 2 house numbers down and didn't know the order they were in. so when they were placed on the house again, they were in the wrong places.
Thats not really a slipup. House numbers can change with out them moving. My dad has lived in the same house for 20 years and his house number was changed.
JerryLee is right I remember that episode Kelly knocks the car into the front of the house or something like that but the point is the house numbers were definitely taken off