The ending to Castle. A series that went on for eight seasons, where they were given several warnings about how the actors (who didn't get along) might quit and challenge production, and then it happens, and instead of preparing a proper ending or deciding to recast Beckett, they had the characters win against the mafia, then randomly die because the writers are absolutely obsessed with cliffhangers, then randomly be brought back to life, then randomly turn it into a Wizard of Oz type of ending with kids we've never seen before, all because they stalled writing an ending until the very last moment. As much as people blame Stana Katic for leaving and throwing a wrench into things, you can't say the writers didn't have some kind of hand in how things turned out. Every possible thing that could've fixed the show was voluntarily ignored.