Films blend genres all the time, creating hybrids of genre that lead to subgenre. Mystery-thrillers have aspects of slasher films. Comedies commonly have romantic subplots that lead plot conficlit. Ther are occasions, however, when such mixtures don't pay off that smoothly.
It can be jarring, out-of-place or downright inappropriate. It can also be intentional, jolting an audience out of their comfort space and thrusting them into a much more grim reality. In The Bedroom's seeming romantic conflict of a young man dating an older divorcee of whom his parents disapprove take a sharp left turn into a revenge thriller about half-way through. But so long as the film's thematics fit within whatever narrative structure has been set up, we're generally willing to go along for the ride.
Here are the occasions that didn't - or did in the wrong way - pay off.