LARS & THE REAL GIRL, SUPER, and IN BRUGES all were marketed like silly lightweight comedies and all turned out to be pretty serious - and in SUPER's case, pretty dark - movies with comic interjections. (That said, I really like all three of those movies.)
I know a lot of people were pissed off by INGLORIOUS BASTERDS, with the trailers making it look like a modern/comedic DIRTY DOZEN and instead it was....well, a Tarantino movie, with subtitles. It's my second-favorite QT movie after RESERVOIR DOGS, but it pissed a lot of people off.
Most of Apatow's recent stuff has turned out pretty depressing, although I don't know if that's necessarily because they were marketed as lighter than they really were or that they were just objectively terrible....