I am rewatching my MTMS DVDs, starting back at the first season.

In the ending credits, when there are clips of the actors in the episode with their title card, Ted Knight's is always the same. He pops in through the WJM doors in his blue sport coat, with a big smile on his face.

Why didn't he do the same credits as the others - with a unique clip from that episode?

Except for the pilot episode, where Ed Asner, Valerie Harper and Ted Knight had different clips, season 1 used pretty much the same credit clips for the main cast all the way through.
The two exceptions were episode 19, "We Closed in Minneapolis", where Ted Knight had a different clip, and episode 21, "The Boss Isn't Coming to Dinner",
where Ed Asner had a different clip. Valerie Harper (besides the pilot), Gavin MacLeod and Cloris Leachman had the same clips all throughout the season.

Starting with season 2 the clips changed in every episode.

Except for the pilot episode, where Ed Asner, Valerie Harper and Ted Knight had different clips, season 1 used pretty much the same credit clips for the main cast all the way through.
The two exceptions were episode 19, "We Closed in Minneapolis", where Ted Knight had a different clip, and episode 21, "The Boss Isn't Coming to Dinner",
where Ed Asner had a different clip. Valerie Harper (besides the pilot), Gavin MacLeod and Cloris Leachman had the same clips all throughout the season.

Starting with season 2 the clips changed in every episode.

I guess I just overlooked the credits, then... I really thought they changed every episode in S1. Thanks for the info!