Movies with incongruous (but awesome) genre switches

A movie which for the first-half is one genre,  and then at a very definite point halfway through the film, changes genre completely.

This might sound like one of those crappy comedy/parodies, but its not…, the trick is both halves of the movie are treated completely seriously. Example:

Romantic Comedy/Horror Film

Tagline: Roger had everything he’d ever wanted, until he met Sally… and then zombies attacked!

The first half of the move is a typical romantic comedy… rich millionaire is hedonistic and shallow, until he finds a girl who refuses to be bought and he has to learn to be humble and care about people, and at about the one-hour-mark, they’re walking through a park, staring lovingly into each other’s eyes, a man is walking up to them in ragged clothing, and Roger in his new spirit of generosity goes to give the man some money, but the man is a ZOMBIE! Sally must flee to Roger’s country manor to escape the growing hoarde, on the way meeting a series of strangers. Then they all die in a dramatic twist ending!