"Surprise! I've been in this room for, like, two hours."
Movies get away with this quite often because the audience doesn't think about it. The killer or Hamburglar or whoever leaps into frame, and you jump and think "Fuckballs! That scared me! I am truly among the spooked!" and then they attack our hapless hero and the action continues. But if you were in the room, that guy literally would have been standing next to you the whole damn time. That's at best. At worst, he had to wind sprint in from down the hall or across the room, giving you and the other characters in the film time to say, "Oh shit, here comes Doug the Stabber. Maybe let's run."
There's a scene in the epically preposterous movie Shark Night 3D in which the heroine has divined that the local sheriff is up to shenanigans, and is sneaking up behind him in her own living room when another of the movie's villains simply grabs her from the side. So unless he is some kind of advanced sofa chameleon who was blending in with the furniture after sneaking into the room through a silent trap door, this dude literally had to dock his giant boat outside, run up to the house, come inside, and cross the room, all the while remaining undetected by a girl standing right in front of him. A girl who, mind you, did not have bleach in her eyes or errant bits of fatty pork shoved in her ears.
To call it the worst part of a movie that includes a shark that looks like this is really saying something.