Gordon throws a dart, and it hits the target's right arm, just above the rectangle at the elbow. When Jane goes to remove the dart, it has moved up higher on the arm between shots.
When the explosion occurs in the mayor's car, the trunk lid of the auto is blown off, and the roof beam of the garage crashes to the ground on the left side. However when next seen, the trunk lid is back on the car, and the roof beam has only partially collapsed.
The powder used in the bomb that killed Mayor Daniels was discovered by Gordon McKay to be the same powder as used in .38 Magnum ammunition. The .357 Magnum cartridge was created by Elmer Keith and manufacturer Smith & Wesson about 7 years prior to this movie. There was, however, never a .38 Magnum cartridge.
The interior scene of the 1940s Chrysler sedan with the principal characters shows it to have a two-piece rear window. Those models feature a one-piece rear window.
In the first scene, the city view with busy street is reversed.
In the final scene, the Bunsen burner can be seen going out. A few seconds later (without being re-lit), it is used to light a cigarette.
Several men are eliminated from the investigation because their hair shows no trace of the dust and fibers at the crime scene. However, as the crime took place several days before, they could have washed their hair in the interim.
Mitchell identifies the killer by testing his cigar cutter. However, as she was not at all suspicious of him, there was no reason for her to make the test.
At the bombing scene, after Gordon tells Gerald that "nothing is ever destroyed, my friend", a moving reflection of the boom microphone is visible in the car's driver's side vent window.
Jane puts her flower into a vase, wanting to display it. But she puts no water in the vase.
Jane puts the end of the wire cut by Gerald's cigar cutter into the putty, so it would not be visible under the comparison microscope. Also, there are at least three major discrepancies between the two examples, especially comparing the lower halves of each.