Two come to mind; Dune from way back in the day, and Apollo 18. Both require explanation.
Dune was just boring. Boooooring. It's basically Star Wars without all the excitement and interesting characters and story. Patrick Steward couldn't save this unfun fucker, and the whole "whispering as internal dialogue" thing was unnecessary. I cut out when the royal family of desert-dwelling pants-pissers were getting all freaked out because the one guy knew how a fucking camel pack worked.
A girl I was dating and I had decided to see Apollo 18 for a date, and it was no longer showing when we got to theatre. No biggy, we went to see Contagion instead. I got a chance to see it later, and I was- well, I was expecting it to be bad, but I was hoping it would be the "so bad it's good" kinda bad, like there was a xenomorph knockoff prowling around on the moon. But no, those little rock spider things? That's it. That's all that's up there. And everyone dies at the end, rendering the whole thing pointless. Oh, umm, spoilers I guess. Even if you know it's going to be bad, it's a fucking disappointment. Just do what we did and watch Contagion instead.
edit: Whoops sorry, Apollo 13 was a different movie. Correcting...