Ending ripped off of Rosemary's Baby
The movie itself wasn't too terrible (albeit still pretty slowly paced) but it was creepy and I felt like the acting was good. However, the ending was just atrocious.
Aside from it being cliche and rushed, it completely ripped off of Rosemary's baby (girl gets pregnant but doesn't know from who, turns out it is the son of satan/demon/whatever to become the antichrist, actually turns out that all of the "good" people in the movie are the cultists, etc).
When the movie ended I was literally like "you have to be kidding me....." it was so promising the whole movie and the whole "was she possessed vs mentally ill" thing was left nicely ambiguous, but then the ending just sent the movie downhill so fast.