Basically, if the movie stands up on other points, like the overall plot, the acting, the characters, the action sequences--I can overlook small blunders. Spiderman II is a good example; there are so many stupid things in that movie, like performing an experiment to create a minature star for the first time in a Manhattan apartment building in front of dozens of reporters with little protective gear instead of in a controlled lab environment far away from population centres. But the acting, character development and action scenes were all so well done I was willing to overlook that sort of thing.
Alternatively, a movie that is dumb but knows it is just a fun, dumb movie like Doomsday, Zoolander, or the Transporter movies--I can forgive all the inconsistencies and irrational things as long as the action is good (not too much shaky cam or bad CGI) or the jokes are funny.
On the other hand, a movie that thinks it's smart or important but isn't--like a lot of Oscar contenders, Jumper, Vanilla Sky--I find they insult my intelligence, so I'm more likely to nitpick every flaw in that case.