Well, there are the infamous ones of the number on the door of Lois's apartment changing (Word of God had it that this happened when the numbers were removed during a season hiatus so that the door could be repainted and then accidently put back on in the wrong order...).
Then there's the constantly changing date of the Senior Kents' wedding anniversary - IIRC we were given at least three different dates in various episodes.
Then there's the height of Lois's apartment - anything from just a couple of floors up to the top of a skyscraper depending on which episode you watch...
That's all that occurs to me at the moment.
LabRat
