Kay this scene takes place when Tatum goes into the garage and sees the killer in there. She throws the beer bottles at him, and if you look closely, you can see the killer's black costume lift up. It is then that you see that the killer is wearing a red plaid shirt underneath his costume. Later, Cindy goes to the door and Billy jumps into the doorway, scaring her. Billy is wearing a red plaid shirt. For this reason, I knew that Billy was the killer before I was even supposed to. This may not be a slip up, but the fact that it spoiled it for me definitely makes me see it as a mistake!