At the beginning, Christina (Cloris Leachman) is shown running at the side of the highway, but the shots of only her feet show her running along the painted center line of the highway.
Mike Hammer approaches a street vendor, buys some popcorn, and walks away eating it (which takes a few seconds). A clock in the background progressively shows 2:00, 2:15, and 2:20.
When Hammer is talking to Evello in Evello's mansion, the telephone receiver in the background switches position between shots.
When Mike opens the "whatsit" box kept in a locker, in the shot made from behind the box, he unstraps left-hand side strap first, but in the shot made from the front of the box - he unstraps right-hand side strap first.
When Velda is reading a magazine in Hammer's apartment, she tosses it into the fireplace under the logs when she gets up to kiss Hammer while he is lying on the couch. Afterwards, when she gets up to leave, the magazine is not in the fireplace.
In the beginning of the film, Hammer stops at a gas station, and the attendant tells him "all I ever pick up is poison ivy." There is no poison ivy in southern California; only poison oak.
Sodium Pentothal is always injected intravenously, but Mike gets it in the calf, subcutaneously. That would cause irritating blisters and tissue necrosis, and probably not the intended "truth serum" effect.
At the beach house, Albert Dekker addresses the character Lily Carver by the name Gabrielle. While similar to the actress's real name, a change in character name is true to the plot.
When Hammer is brought to the beach house for the first time by the two thugs and is trying to escape the ensuing brief chase, the fight is shown in alternating close ups and from distance. The shots from distance clearly show different actors, stand-ins for the thugs, than those playing them in the close-ups and throughout the movie.
About halfway into the movie, Mike checks his answering machine. A running tape on a tape recorder tells him 'There have been no calls during your absence.' How could this be on the tape? Even a modern answering machine would not know that he'd been absent.
(at around 1h 10 mins) The singer is shown standing with both hands on the microphone stand. When Hammer is sitting at the bar, during the same song, the singer is shown in a reflection playing a piano and not singing, though she is still heard singing.
When Mike pulls into the gas station and asks to have the right front wheel checked, the attendant says, "Yes, sir!" but his lips don't move.
Hammer pulls into Carl's (Paul Stewart) driveway. Friday (Marian Carr) meets him, and when he gets out of the car she shuts the door, but there is no sound of the door shutting. The audio and dialog must have been added in post-production, and someone missed the closing car door.
The shadow of the camera is visible on Mike Hammer in the tracking shot of him entering and walking into his apartment.
When Nick discovers Hammer's Jaguar XK120, a crew member's reflection is visible in the driver-side door.
After talking to Nick, the Greek garage owner, Hammer enters a taxi; as it begins to move (16:39), a crew member with a baseball cap is reflected in the rear quarter light window.
When Mike Hammer is talking to the old man with the trunk in the doorway of the apartment house, a microphone is briefly visible in the upper-left portion of the archway.
When Mike leaves Christina's apartment the old man is fussing with his effects at the curb. As the camera dollies in, the man glances up straight at the camera.