At the end of the movie, Brenda, the killer, is shot by Natalie. Right as Brenda is shot, she is blown back by the force of the shot into the window behind her. If you will notice, the window does not, nor did it, ever have glass on it. It is simply boarded up with wood. When Brenda is blown through the window you can clearly hear the shattering of glass although there is never any glass in the window.