i don't know how many others have seen these mistakes also, but i find them really annoying! to me, after the 4th season, the writers did not seem to watch the earlier episodes, and completely changed things around. here are some mistakes i've seen:
In the episode where mr. drummond decides to spend the night with a woman while the rest of the family go camping, he is nervous to reveal to them that this woman is his girlfriend, fearing arnold would not understand that sort of thing. well HELLO!, drummond had many girlfriends before, and arnold had even helped him get back together with a hairdresser in an earlier episode. later on in this same episode, drummond sits down to talk to arnold, and talks about how he and arnold had the discussion about the birds and the bees. another major mistake! willis had actually told arnold about the birds and the bees in the first season, and judging from the face of mr. drummond when arnold tells him what willis had told him, it must have been quite detailed! this same discussion actually turns up again in the bicycle man episode, where arnold says that drummond had taught him about the birds and the bees. in reality, it does make more sense for a father to teach this than an older brother, but there is no reason why the writers changed these things around. in every tv show, storylines should follow earlier episodes, so that things make more sense. does anyone else here agree?

In the Pilot for the Facts of Life , the school was called East Lake
not East Land. What a weird name for a girls school Lake??????

That one of my pet peeve errors in the show.

also when the arnold character appeared on silver spoons, he said he was a school newspaper reporter but there was never a episode of different strokes with arnold being a reporter

Diff'rent Strokes had more regular errors in it that any other sitcoms I know.
Willis and Kimberly's ages were constantly swopped around at the beginning, Kimberly calls Willis "big brother" in the pilot but he is later called "little brother."

Also, in the episode "Roomates" Drummond said Arnold moved in when he was 6, which is of course wrong since he was 8.

There are others that I can't remember right now.

what about the doll with 2 names in the goodbye dolly ep. from season 1 the dolls name is homer and then in the ep in year 7 where sam wets his bed arnold tells sam that he once slept with a doll but he said the dolls name was fo fo.