A question over on the Limerick forum just reminded me of this one.
Was over at my girlfriend's then apartment two years ago and a few of us were settling in to watch Aliens (or possibly Alien, can't fully remember), as the opening credits rolled and we got a shot of a spceship floating through space one of the girls turned and asked 'is this a true story?' :D
Remains the single most dumbest thing I've ever heard, although in fairness to the girl as soon as she said it she buried her face in her hands and stayed quiet during the laughter that followed for the next couple of months