Game of Thrones
Game of Thrones was a show famous for its high number of characters, so it made it all the more confusing when Sir Beric Dondarrion was replaced out of nowhere. David Michael Scott played the role for only one episode before being recast by Richard Dormer. Amazingly though, that wasn't Scott's only day on set. He returned to play Beric's brother in a later season. It was very confusing for those with a great memory.