I've noticed that the song credits at the end of the episode sometimes list a song that wasn't in the episode, and neglects to list the song that was.

Q. Did they later (for syndication) change the song that had originally been in the episode? If so, why? If not, why the erroneous credits?

I've noticed that the song credits at the end of the episode sometimes list a song that wasn't in the episode, and neglects to list the song that was.

Q. Did they later (for syndication) change the song that had originally been in the episode? If so, why? If not, why the erroneous credits?

They would switch songs for summer reruns, to bring the show up to date with the latest singles and album releases, so it's possible you saw a rerun with a song switched.