Revealing mistakes: When Perry Smith is sent through the gallows trap at the end of the film, it is apparent that the rope is connected to the stand-in mannequin's middle back, rather than around its neck.
Anachronisms: Many vehicles present in film that were not yet introduced. Vehicles from the 1960s during the events leading to the 1959 crime
Factual errors: In the court, the quote from the Bible is actually Genesis 9:6, not Genesis 9:12.
Miscellaneous: During Perry's daydream fantasy sequence, he is seen playing a Saturn 63 guitar. The number signifies the year of the guitar's production, 1963, yet this scene was supposed to have taken place in 1959.
Anachronisms: When Dick and Perry are driving through Las Vegas they pass a billboard for a casino that has Chita Rivera headlining, a highly-unlikely thing in the late 1950s, but very plausible in 1967.
Revealing mistakes: At the beginning of the movie, after Dick has picked Perry up from the Kansas City bus depot and both are crossing the river into Kansas, the process is running in reverse, giving the impression the car is suddenly moving in reverse despite the two immediate shots bookending this one clearly shows the car moving forward.
Miscellaneous: When Perry and Dick arrive at Clutter's Place, still sitting in the car, Perry says: "Let's pull outta here". Then Dick drinks from his bottle. Later in the movie, when Perry makes his confession, we see the same scene, but now Dick drinks before Perry says: "Let's pull outta here"
Crew or equipment visible: When Dick is in the interrogation room just before he confesses, you can see the reflection of the camera in the window on the door before and after the detective enters the room.