Executive Meddling: Executives at CBS prevented O'Connor from filming a true series finale. O'Connor was reportedly so angry that he wouldn't work with the network for a decade afterwards.
McLeaned: Jean Stapleton left the series after the first season due to feeling she'd gone as far as she could go with her character. This led to Edith dying off-camera between seasons and was the premise for "Archie Alone". Unlike most examples of this trope, Norman Lear actually did this to give Edith a dignified end and apparently took the death of the character personally.
What Could Have Been: According to a 2001 E! True Hollywood Story special, if the show had continued for a fifth season, Mike would have returned for a cameo, and the show likely would have had (according to Carroll O'Connor) a proper series finale.