"The Simpsons" is notorious now for its mediocre endings, but I think the one that started the trend was 5F07 "Miracle on Evergreen Terrace". I like dark humor as much as the next guy, but the entire town of Springfield stealing everything that the Simpsons own just didn't do anything for me. The episode in itself is extremely dark, and from the moment in Act 1 where Bart burns down the Christmas tree, it just spirals further and further into a pit of depression until it finally hits bottom at the end. Seeing the Simpsons sitting alone in their empty living room with nothing left but a washcloth doesn't make me laugh - it makes me feel depressed, and just a little bit angry. Not at the fact that the Simpsons will inexplicably have all their posessions back next week (I expect that sort of self-referential goofing from the show), but at the fact that the writers thought they could get a laugh by treating the family like crap over a matter that they didn't even have any control over. It was all Bart's fault, yet Homer, Marge, Lisa, and Maggie have to suffer too, even though they were victims to begin with. The people of Springfield are stupid, but for God's sake, they're not supposed to be heartless (not all of them, anyway).
If Bart alone had received the consequences of lying, then it would have been an okay episode. But the rest of the family didn't deserve what they got.