On the pilot for Living Single, they use the living room set from Family Matters for the ladies' apartment. The front door leads directly outside, as opposed to in all the following episodes, where it leads into a hallway that includes a staircase that goes up to Kyle and Overton's apartment. Also, Overton is very stupid in the pilot and the next episode that was filmed (which aired as episode five), but thankfully not in the other episodes.
On the first aired episode of A Different World (not the pilot, and there seem to be at least a couple episodes filmed even earlier, with one just dumped into summer and another with one storyline cobbled together with another involving characters introduced later to complete the show), Whitley mentions something about her sister, but she doesn't seem to exist in any other episode.
Eric McCormack's hair in the Will & Grace pilot is I think much longer than in every subsequent episode.