When Buddy talks to Guy about producing Dawn's movie, the script has "Foster Kane" written on the front in big letters. But earlier in the scene, when Buddy is carrying the script into his office, there's nothing written on the front of the script.
When Guy and Dawn are in the launderette, Dawn's collar alternates between being up and down.
During the "Sweet and Low" scene, the packs of Equal and sugar continually swap places.
Guy's head is alternately up/down between cuts when he is holding Buddy at gunpoint.
Near the end of the film when Guy is in the office and his friend asks him to put in a good word for him, Guy stubs out the same cigarette he is smoking, in 2 separate shots.
When Guy imitates Buddy in his office, Buddy suddenly appears and asks him what he's doing. Guy responds with an evasive, but Buddy does not complain. Taking into account Buddy's personality, it is clear that he believes that what Guy is saying is true. However, it is not possible for Buddy to have entered the office and become so close to Guy without listening to what he was saying.
When Dawn loads Guy's washing machines at the Laundromat, we never see either of them put quarters into the slots. Then, when Dawn shoves the handles in to start the cycle, we hear that there are no quarters dropping into the bin.
When Guy is recalling his favorite movie memories on his first date with Dawn, he recalls the summer of 1988, in which he saw Indiana Jones and the Last Crusade, a movie that was released in 1989. Then again, Gabe Kaplan was not in The Fish That Saved Pittsburgh (1979) as Guy says he was, so it's clear that his memory isn't all that he claims. He was probably thinking of Fast Break (1979) in which Kaplan played a basketball coach.
Guy puts a CD in the player upside-down. The display then shows "no disc" but the music begins to play anyway.
According to the movie guy and Buddy tell the police that Dawn did the torturing and Buddy saved him by shooting her. But Guys fingerprints would have been all over that house and Dawn's would have been nowhere. Even a rookie cop could have seen through that BS story. Bullet holes in the ceiling, bullet hole in the telephone.
When Guy relates the story of Shelly Winters' interview with Buddy, he states that Shelly takes three Oscars out of her bag, when in fact she only won two.