When "Castle" was still airing, it lost me a couple episodes into its 8th (and final) season, because the writers made a baffling decision and handed one of the leads the Idiot Ball for, like, ten consecutive episodes. I went back and rewatched it recently, and watched the series finale last night, and boy do I have some things to yell about. That last season was a fucking mess and the finale was terrible.
@noelle But for me the absolute worst was The Walking Dead. Started so well, then they continually wrecked it.