The original Eternal Sunshine ending. I can't find a link for it, but the original text of Charlie Kaufman's screenplay for Eternal Sunshine of the Spotless Mind is floating about. Too bad the original ending was not used and we were given more of a Hollywood conventional ending. Maybe they filmed the original ending and we'll get it on the DVD, so don't read on if you don't want it spoiled. [UPDATE IN BRACKETS: Chris tells me the original ending was filmed, but it tested poorly, so expect to find it on the DVD. If you want to wait for the cool surprise ending, skip the next sentence.] But in the original screenplay we see the two main characters as old people and learn that they have gone through their whole lives erasing each other and then getting back together fifteen times!
Another Hollywood decision that was imposed on the film was to concentrate on the two main characters--"This is really Joel and Clementine's story..."--and only use the other characters to further the plot. You may have noticed that the Jim Carrey character referred several times to his current girlfriend Naomi, but then she never showed up. She is in many scenes in the original screenplay.
One thing in the movie that bugged me was that the way the Jim Carrey character is suddenly inspired to skip work and take a train out to Montauk and go to the beach on a cold day. He wonders what came over him. Then, when there's one woman out there on the beach, and there she is again in the restaurant later, and then again on the train home, it never occurs to him to connect her insistent presence with his original impulse to go out there. Wouldn't anyone with half a brain have thought fate made her appear here? I mean, if he did that first impulsive act with this whole "something was drawing me off the one train onto the other train" attitude shouldn't that belief in destiny have stayed with him so that he would expected something to happen? And then she's so obviously it. But it would be dumb to have him just see her as his destiny at that point, so he can't say it, and we're not supposed to notice.
UPDATE: Trimblog has a link for the original screenplay.