Here's the first five to pop in my head since there's so many:
Bob and Kathy (Willy's Wonderland) - Recent, yes, and I like this movie, but these two having random sex in the middle of a killer animatronic carnage is beyond idiotic
Rebecca Chambers (Resident Evil 0) - Why doesn't she leave the train and get back up at the beginning besides needing to have a game?
Liz (Wolf Creek) - Never abandon a killer's weapon for god's sakes
Micah (Paranormal Activity) - I already hate these movies but maybe it's not a good idea to taunt the paranormal specter haunting your house
Everybody in Tucker & Dale vs. Evil - But it's by design so it's fine!