The outside of the sorority house says pi kappa sigma. A picture hung inside the house says pi beta phi.
Peter was outside the sorority when Jess was attacked and hid in the basement. It makes no sense that he would know to look for her there.
Before the phone call from Barb's mother, her shirt has three buttons undone, then after the call, it has one button undone, and then there are three buttons undone in the next scene.
When Clare Harrison enters her room there's no Christmas decoration on the door. Afterwards, a bow shows up on the same door.
When Claude the cat is first seen, during the first malicious phone call, he's rather short-coated, but later he's become conspicuously more shaggy.
Mrs. Mac continues to scream in pain even after the hook stabs her neck and yanks her into the attic. She wouldn't be able to mutter any noises other than throaty gags or gurgling if the hook pierced her windpipe or throat.
Barb is clearly not pushing down on her inhaler as she's breathing in the mist.
Alluding to the intention by Jess to go for an abortion, the obscene phone caller taunts "like removing a wart," the exact words her boyfriend used, and then the line goes dead immediately afterwards. Olivia Hussey then gasps "Oh, my God" and cradles the phone. In the next scene, Lieutenant Fuller wants to know why she got so upset and uttered those three words. But the line had been disconnected and her response could not have been listened in on. However, it was set up for him to be able to listen in on her phone, essentially an extension phone. While Billy hung up, Jess still held the phone's receiver in her hand. Anyone listening on an extension would hear any noise or conversation, which is why he was able to hear her reaction.
The hook that impales Mrs. Mac is clearly not sharp enough nor heavy enough to penetrate skin.
Jess, an American college student, has a noticeable British accent as a result of being played by actress Olivia Hussey who, although Argentinian, was raised in Britain. However, Jess could have been raised in Britain to the point where an accent could have developed before moving to North America with her parents.
The slide for the chain on the back door of the sorority house is mounted vertically, not horizontally as it should be, making it possible to just reach inside and slide it up to open it.
One of the times Jess answers the phone, it continues to ring for more than a second after she picks it up.
When Billy first enters the attic through the window at the beginning of the film, his shadow reflected on the wall clearly shows the camera equipment he is carrying.
When Clare enters the closet to check Claude the cat the camera points to Clare and you can see moving in the background to her right - a crew member and cord.
At 5 Minutes When Billy Is Climbing Down The Ladder You Can Clearly See To The Right Of His Hand The Circular Lens Being Shadowed On The Wall. This Happens Again When He Comes Down The Ladder Again Later In The Film.
Barb drinks a can of King Kan beer at one point. This brand of beer is not sold in the United States, where the film is set, but rather in Canada, where it was actually filmed.
Before the search party begins, a man wearing a Canadian beret is visible in a car door window reflection when Mrs. Quaife is looking worried, even though the film is supposed to take place in the USA.
People in the USA, of course, are not allowed to wear Canadian berets.
People in the USA, of course, are not allowed to wear Canadian berets.
Though the film is supposed to be set in America, several characters in the film noticeably speak with a Canadian accent, the film's actual shooting location.
One of Mrs. Mac's high heels fell off after she was dragged into the attic. This suspicious piece of evidence should have served as a clue that something's amiss in the attic to the police upon their arrival but it's never mentioned again. However, Mrs Mac was alone in the house when Billy killed her, and after he calmed down he could have gone down the ladder and picked the shoe up. This could have happened long before anyone, including police, might have come into the house.
Shadow of boom mic and reflection of camera equipment visible on wall and pictures during the last tracking shot of the movie down the hall.
About seven minutes into the film, when "the moaner" calls, you can very clearly see the boom mic at the top of the frame as everyone gathers around the phone to listen.