Dark Angel: When Producer James Cameron came in to direct an episode. I'm a big Cameron fan but that episode was the worst of the series and it went downhill from there.
The Cosby Show: When the older cousin shows up.
Firefly: I actually think that the movie killed it. All the nutzo fanbase drove more rational people away from the franchise. I love Firefly dearly but those Browncoats bee craaazy. The movie came out. Die hards had seen it a couple of times at free screenings. The name was changed confusing some. Nutzo internet Browncoats scared normals. Movie underperformed. Franchise REEEEaly dead.