5. Becky - Roseanne
The notably eccentric Conner family on Roseanne featured three children: Becky, Darlene, and DJ. Their working class exploits were a refreshing change from the more white collar family sitcoms we had been used to watching on television throughout the 80s. But as it often happens when shows feature teenage actors facing major life decisions, Roseanne had to be restructured after Becky actress Lacy Goranson left the show to attend Vassar College. For a while, they had her character merely absent, off living with her husband and just not appearing in the episodes. But at the beginning of the sixth season, they brought on Sarah Chalke (of Scrubs fame) to take over the role. If it had ended there, it would have been fine -- just a run of the mill casting revision. But strangely enough, they brought Goranson back whenever she was available to film, the practical result of which was that the two actresses would sometimes appear in alternating episodes. It sort of worked, though. Roseanne was known for its meta sense of humor, and having two actresses play one of the main characters in the later seasons was just one more delightfully odd production decision.