Continuity mistake: When Laura chases a missing Simon to the cave she injures her right leg. In the hospital her left leg is bandaged. (00:33:50 - 00:34:55)
Continuity mistake: The amount of sacks by the oven that Laura moves away keeps increasing and decreasing randomly all the time.
Continuity mistake: When the medium leaves the bedroom following the first countdown, she turns right out of the room, but then 'backtracks' on herself, passing the room she had just left (you can see the chair she was sitting on in the background). However, Enrique, on his house plan, marks her walking in the direction she first took (right) and not as if she had turned left out of the room.
Continuity mistake: When Laura discovers the basement door, a yellow iron pillar falls against the door. In the following shot the pillar is back on its place.
Answer: It was Simon. He had found the mask in Thomas' little house and attacked her while wearing it because he was angry she had fought with him. We know it is him as the boy in the mask is wearing Simon's shoes, and later on when Simon is found he is wearing the mask.