In this scene there are actually two slip-ups.

This scene is when the dark haired lesbian is beating up the killer, the scene starts 01:08:00 or something like that.

When the killer falls to the ground and she goes on top of him there is nothing above his head. Then she starts beating him and her face is shown. Then again the killer is shwon and a coat hanger has appeared from nowhere. Then they show her beating him up a bit and after a bit the coat hanger dissapears again and suddenly reapears.

The other slip-up is when the killer pushes the girl off of him. When he does that his sleeve falls back and a white regular hand appears but the person was supposed to be severely burned all over his body, you can even see his hands burned in the last scene.