Dawson Casting: Absolutely no effort is made to have any of the actors playing "high-school age" characters look like they're anywhere close to their supposed ages.
Doing It for the Art: Chris Pratt was so eager to work on the film version that he lied and said that he lived in New York because the production only wanted to hire local actors to save money. He ended up losing 3000 dollars during the production because he had to pay for his own lodging and, at one point, had to fly back to Los Angeles to deal with an insurance ticket he forgot to pay.
The Other Darrin: They had to replace Jerri's brother in The Movie because he "looked like a lumberjack". Also Jerri's dad bares no resemblance to his actor in the show whatsoever.
Script typos such as "I'll be back in a shortly" and "This was this one time..."
Torch the Franchise and Run: The finale has shades of this; the series ends with Flatpoint High being destroyed by students and faculty alike so it wouldn't be converted into a strip mall, leaving no way to continue the series. The Movie got around this by being a Prequel.
You Look Familiar: Virginia Williams played four different characters throughout the series. It would seem at first that Delores Duffy did this, too, playing the school secretary Iris Puffybush, a nurse, and the owner of an artificial flower factory in various episodes, but it looks like she's really supposed to be all those things at the same time.