Entertainment Weekly has a slideshow of movie errors that bother people. I have to say, most of them are quite minor, and the sort of thing very few people would notice. (The one about Pi did bother me, though.)
But I guess we all have different things that annoy us in movies. I never did understand why the giant laser guns in Revenge of the Sith seem to be ejecting casings. That makes no sense.
The best errors, though, usually come in movies about some historical event, like people wearing wristwatches in the movie Spartacus. The most glaring examples I can think of come from the movie Battle of the Bulge, in which the German tanks are actually American M47s. Even more jarring is the fact that the final stage of the battle appears to be fought in a desert. There is, as far as I know, no desert in the Ardennes.
But some people probably wouldn’t be bothered at all by things like that. For one thing, in the EW article, a lot of people mentioned being bothered by characters going in the wrong direction to reach their supposed destination. I have a lousy sense of direction, so I would never notice that kind of thing.
What kind of movie errors irritate you?