For example on Friends, compare Chandler's appearance in the Season 6 finale (https://friends.fandom.com/wiki/The_One_With_The_Proposal,_Part_2) vs. the Season 7 premiere (https://friends.fandom.com/wiki/The_One_With_Monica%27s_Thunder). It's supposed to be a continuation (https://www.moviemistakes.com/tv2651/episode154) of the same scene, but Matthew Perry lost like 20 pounds in-between:
https://78.media.tumblr.com/tumblr_l6zex0ReAV1qzlhqfo1_400.png

https://i.imgur.com/wz3HAxS.png

In The Donna Reed Show original airing from 3/14/1963, Bob Crane plays Dr. Dave Blevens. Three weeks later (OK, not the next, but close) on 4/4/1963, he settles into his regular role for the rest of season 5 thru 7, Dr. Dave Kelsey.