The Chuck Cunningham Syndrome

You learn something new everyday.  Today, it is the Chuck Cunningham Syndrome.  From Wikipedia:

He [Gavan O'Herlihy] was cast as the eldest sibling, Chuck Cunningham, on Happy Days.[4][5] He played Chuck during season 1 until the episode "Give the Band a Hand" and was replaced for season 2 by Randolph Roberts until the episode "Guess Who's Coming to Christmas". Chuck was not seen again but was later mentioned in a few other episodes ending with "Fish and the Fins". After that, Chuck was written off the show completely with later episodes depicting the Cunningham family with only two children with Richie as the elder. The character gave rise to the pejorative term "Chuck Cunningham Syndrome", referring to characters who disappear from TV shows without an in-universe explanation and are later retconned to have never existed. O'Herlihy did not want to become stuck in television roles, preferring films.