Nailing the last episode of a show takes a lot of work, and whilst some may not fully hit the mark, some have entirely ruined the build-up in a show.
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Alf (1986 - 1989) had a depressive, disappointing ending. The movie that followed doesn't count for me. They really messed up what was actually fun series.
I'll Submit my answer.
Game Of Thrones. I loved the show (Still do) and the first six seasons were some of the best television. However the 7 episode long season 8 failed to hit the mark. It was just disappointing for a show that was doing so well.
It would have been ok (Dany going bad), but the transition was too fast, too rushed. And Bran on the Throne...no, just no. Because he knows stories? C'mon! S7E3 - Bran: "I can never be lord of anything. I'm the Three-Eyed Raven.” Bran in the Finale: ''Why do you think I came all this way" It's just not right...anybody but him.
The final of Firefly..... it was axed so never wound up anything.... at least the movie tied up a bit of the story.....
And Angel.... just had to rush an ending cause it was cancelled.....
Little House on The Prairie, when the townspeople just decided to blow up Walnut Grove at the very end of it all.