Season 10: The one without Ellen
The TiVo had barely gone cold on Ellen's debut when she packed up her vest and sneaks and said her goodbyes in season 10, claiming the show was not the "right fit" for her (read: she was too nice to crush people's hopes and dreams).
At the same time, Kara DioGuardi's contract wasn't renewed, meaning there were two, possibly three, spots to fill in the dragon's den. They eventually went to someone called Jenny from the Block and a dude who looks like a lady - that is Jennifer Lopez and Steven Tyler, making it the first season with only three judges in quite some time. (Randy Jackson remained and will stay to haunt the studios until the end of time).