When Frank visits Helen in the shoe store, she takes his shoe off. He never puts it back on, and he leaves the store in a huff.
Signs say "8th Annual Yankee Brunch" but voice on PA says "3rd annual Yankee breakfast".
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 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.
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.
Al Pacino's character's name is Frank Keller, yet when he is in Helen's apartment and sees his singles ad on her fridge, it clearly says "Frank Kellog". However, this is not a mistake. A police officer would not use his real name in an undercover operation.
When Terry slams Frank against the mirror in Frank's apartment, we hear the sound of breaking glass, yet later in the scene the mirror appears intact.
Mirror bending as if it was made of rubber, when Helen's husband is pushing Frank against it.
In the scene where Helen and Frank are walking down the street at the end of the film, the camera and sound crew are reflected in a shop window.