11. Seinfeld, Season 8 (NBC; September 19, 1996 - May 15, 1997)
Sometimes one season of television can turn bad with merely a single episode. Many fans feel that Seinfeld's finale season was lackluster overall, as comedy mastermind Larry David had already left the show. What they likely remember most vividly, however, was just how dissapointing the finale was.
The "show about nothing" ended with a strange reunion episode, as the characters finally received their comeuppance in court, at the hands of characters from their past, and managed to land themselves in jail. Anyone who was watching TV at the time remembers the extreme hype around the episode, and perhaps we are less kind in retrospect because of the build-up.
Nonetheless, a finale that was both structurallly and geographically different from the formula that made the series great was unexpected, and enough to tip a middling final season into flat-out dissapointment.