I'm really surprised (well, not really with the heated but superfluous LOTR "ending" discussion) that no one mentioned . . .
. . . Lost In Space
That movie had EVERYTHING going for it - good cast and acting, good special effects, a cheesy TV series to take off from with a decent plot, and then - WHAT was that last 10 minutes??? . . . Dr. Smith turned into a spider and they went through the center of the planet to go back just a little in time for no advantage whatsoever other that to horrify and shock the audience.
Disgusting ending . . . one of the worst ever.
"Nightfall" was just the worst ever . . . the ending matched the rest of the garbage (so sad, especially considering the Asimov story of the same name).