Failure to stick the landing has been a very common problem with very good series. Dexter, Scrubs, and Gilmore Girls comes to mind. Not that it’s a “good” series, per se, as it was often hit or miss, but I was a diehard fan of Smallville and was incredibly disappointed in the last season and the finale, too. I’ve also never finished Battlestar Galactica’s last season because I’ve heard it leaves a bad taste in people’s mouths. And I never even picked up watching Lost because of its notoriously unsatisfying ending after its also notoriously addictive mystery breadcrumbs lead to nowhere. It applies to a few novel series from my childhood too, like the Pendragon series, and The Inheritance Cycle. It sucks when you devote years following a series, loving the characters, plots and mysteries, only to end up not ever wanting to consume it again because of the massive disappointment in the end.