Oh do I have the movie for you. "Tucker and Dale Versus Evil".
You know how in horror movies the characters are always oblivious to all the signs that, well, they're in a horror movie? Turns out that the opposite can be just as bad: a bunch of stupid college teens mistake a pair of friends (Tucker and Dale) for killer hillbillies out for their blood, and their increasingly desperate attempts to save themselves from the horror story they think they're in (and some truly bad luck as far as miscommunications) end up killing them off one by one, as Tucker and Dale try to figure out what the hell is happening.
EDIT: Or for something cornier, check out "Knights of Badassdom". A group of LARPers manage to summon an actual succubus from Hell. Oops.
And I'm sure somebody's already mentioned Sharknado, but if not.... Note that you have to make it to the end, because the ending really does save the rest of the movie, changing it from "so bad it's bad" to "so bad it's awesome".