That could be a thread all on its own.
For me, Peter Jackson's King Kong, as it was shit and we knew we could probably catch the start of another film; same goes for Shrek (although I like the sequels, oddly). There's another one I've forgotten, and I wished I'd left Congo early once it was over. Aside from King Kong, the wife tells me she walked out of the 1998 American Godzilla, and I can't say I blame her.
Anyone seen memorable walk-outs from other people? A sizeable percentage of the audience started bitching and walked out of Brotherhood of the Wolf en masse when the subtitles kicked in at the beginning, and practically every late-night horror movie I ever attended in the States would feature an early walk-out from a couple after the baby they brought with them started screaming in terror