During Oliver Keane's accident scene, he is seen driving a gray car at first, but when he skids off the road and tumbles down the hill, it is black.
At the beginning of the film, Althea (Audrey Totter) is shown removing the pay phone receiver from her ear twice between shots.
The position of the hotel's sign outside the window changes between shots. From the outside, the letter "K" is even with the top of the window. When first seen from inside, it is lower. But in the next shot it's back higher again, even looking from the same level - so it spells out "KILL" to the resident of the room.
When Grandison pours the glasses of champagne, one of which is poisoned, bubbles do not appear in either glass. Then in a set piece with the glass containing poison in the foreground, it is full of effervescence. Then when the two glasses are seen again, no bubbles.
At the party, when Althea holds the match out to light Steven's cigarette she is away from him and facing the camera in the first shot, but in the next shot she is much closer to him and facing him.
The radio station studio microphone shows periods after each of the initials, WMCB. Actual radio station or radio network studio microphones had no period after each identifying initial.
To compare the grooves in the bullets made by a gun, the point of the bullet is stuck in the clay, not the area where the grooves are.
As in so many run-away, car-without-brakes scenes of the 1930's & 1940's every driver "forgets" that the automobile has an ignition that can be turned off with the key. It can also be down-shifted to further slow the careening car.
Donovan gives Howard's birthday as April 12, 1913, but his driver's license has 1-12-1922.
At about 7 minutes in, 3 envelopes addressed to Grandison are shown on his desk. On the third, the name is mis-spelled as "Grandisn".
A character arrives at the party, and the door is opened by the manservant; the door closes, and the window reflection shows the cameraman and, likely, Director Michael Curtiz.
When Steve Edwards shows up for the first time at the front door of Grandison's home, the cameraman is clearly reflected on the window of the door.