Unintentional masterpiece
Since the director/writer did not produce much before (or after - but irrelevant here) the film, I am not sure they intended In Bruges to turn out so good. But the movie is absolutely brilliant. The actors are fantastic. The silence, the looks, the facial expressions, so many subtleties that makes this movie one of a kind. From the story you wouldn't expect it to be that great but it is.
Can you think of another movie where the producers probably did not anticipate the level of genius the final film would turn out to be? An obvious example I have in mind would be The Big Lebowski.