At least they included it in the Extended Edition DVD of ROTK. Better than nothing, I suppose.
Gone with the Wind should have ended right after Rhet Butler says, "Frankly my dear, I don’t give a damn.’ and walks away into the mist
For me, the worst part near the end was when they trotted out that horrible, terrible, trite, needs-to-be-banned-forever cliche of the person rushing to save him and having car trouble.
My nominee: Twelve Monkeys. As with the above, the ending was a tired, worn-out cliche, straight out of “Time Travel Writing 101”, the ending that anyone who’s ever watched or read more than one time travel story would be thinking throughout the movie, “Nah, that’s just so trite and obvious, they couldn’t possibly be going there”. But they did. Ooo, it’s his older self in his childhood memories. Ooo, he actually caused (or allowed, whatever) the plague he was sent back to prevent. Made the entire movie (which otherwise wasn’t bad) totally pointless.
Heat.
I always thought it should have been Pacino who dies and not De Niro.
Let’s get some fusion going in this thread, and say that it would have been better if Rhett had walked away into The Mist, where he’s eaten by an extradimensional tentacle monster.
That’s more a problem with Time Travel stories, though, isn’t it? There’s only a few ways to end a time travel story without breaking logic in half, and once you’ve seen them all, they get kind of predictable. I know I wouldn’t go into a time travel story expecting to be surprised anymore.
Such a manipulative lie. Pissed me off.
But you had to see it coming from miles and miles away.
Ding ding ding… I think we may have a winner.
I can’t remember it - oh wait. I just did. Holy mother of god.
The thing that made me crazy in that movie was the ridiculous “I can’t be around cigarette smoke” scene. Are you fucking KIDDING?
Yeah, I think that was supposed to be her “I’m OK with becoming a mommy” scene but still, WTF, priorities. Besides, I’m pretty sure that stuff getting vaporized outside and sucked into the ventilation system (as long as that was still running) would be worse for you.
I agree, I was vehemently disliking much of Blair Witch until that scene, which was really effective for me. Prior to that I was loathing the characters fairly deeply and merely annoyed with them.
I’m gonna say the original movie of The Producers, although it’s not so much the ending as the whole third act. The movie is brilliant right up to the point that they discover that the show is a hit, and then…the inspiration vanishes and the rest of the film is just a quick and dirty wrap-up, completely with comedy vaudeville drunk sitting on a dynamite detonator.
This was fixed in the musical version - the ending, while still farce, was at least consistent with the rest of the show - but without Gene Wilder it just wasn’t the same.
Heaven’s Gate is widely acclaimed to be a big disaster, sure. Wild West, 1870. Cattlemen vs. a group of immigrants horning in. Shooting. Blasting. Battles. Hero falls for a madam of a bordello. Horses. Dust. At the end, our hero is on a yacht some years later. A woman on the yacht (I don’t believe was even in the movie at all) asks him for a cigarette. The. End. 
I haven’t seen this movie in decades, and there may have been a more fitting ending re: the cattlemen vs. the immigrants. This dumb last scene seemed just tacked on (maybe because there was a yacht available and they wanted to use it). If it’s germaine to the story, please correct me.
IIRC, they just hold hands. I wanted that kiss, damnit! I was a teenager when the movies came out, so that’s my excuse. 
I’m no big fan of Twelve Monkeys, by any standard, but I have to defend it on this point. Whether you’re aware of it or not, 12M is a remake/expansion of a classic French SF film La Jetee – one of the earlier time-travel films (despite being made circa 1960), back when this cliche hadn’t been done to death in cinema. Of course they were going to end it that way – it wouldn’t have any point if they didn’t. They’d be making a completely different, non-remake movie. it makes as much sense to fault them for this as it would to fault a remake of Gone with the Wind for including the line “Frankly, my dear, I don’t give a damn!” because it had been used so many times.
I loathed everything about it, the camera work most of all.
The ending of The Bad Seed was changed because of the Hays Code.
'Splain? Did the brat fail to get her comeuppance? It’s been a while since I’ve seen it.
Though it’s not precisely in the spirit of the thread, I’ve always been grateful to the Dope for keeping me from seeing The Village, which I’m sure I would have hurried off to eventually. The ending is the big reveal and precisely the plot, but I think I might have killed something if I’d of had to sit through it.
Pretty sure that in the book, the mother tries to off the kid with some pills, then kills herself in a more decisive way… kid survives, mother does not.
In the movie, I think the mother tries to kill herself, survives, but God strikes the sociopath kid down via lightning bolt.
Edit to add - or check the wiki link given by Superdude. I just remembered it was censored because the atmosphere of the times wouldn’t allow the psycho kid to go unpunished or unchecked.