At her first visit to Franck's apartment, Helen goes to the bathroom and returns with a bathrobe on. When they kiss, it can be seen that she wears her panties. A few seconds later, when she drops the bathrobe, she doesn't wear any.
At the beginning of the movie, the camera pans up to a 7th floor apartment window which is revealed to be that of the first murder victim. Yet later, the apartment is referred to and shown to be on the 18th floor.
(at around 1h 06 mins) When Helen says "Look, why don't you come over to my house tonight for dinner...'' you can clearly see Frank's right hand reaching for his right shoe to lace it. Then you see Frank's right knee when he is putting his leg down.
In one scene as Frank is getting out of bed, he is shown putting on his underwear. But as he gets up to pull them all the way up, you can tell that he already had them on before he got out of bed. You see the elastic band.
The goof items below may give away important plot points.
When Frank throws Terry out the window, Terry goes out with his right shoulder down, facing the building. In the next shot, he is on his back, falling parallel to the building. When he hits the awning, he is now perpendicular to the building, his head toward the street.