I never watched it, but I've heard the 1980s police-procedural-spoof Sledge Hammer! didn't expect to be renewed at the end of its first season, so they ended it with a nuclear bomb destroying San Francisco; then they got a second season.

Hee. Millennium's second-season finale killed off the world and most of the characters with an apocalyptic plague. I have never watched the third season, because it is obviously either an alternate timeline or wishful thinking.