When the Big Clock is first shown and the camera pans down to the lobby floor with the tour guide, it shows that there is a ledge directly underneath the clock which would make the display of its time impossible to see from the lobby.
George Stroud introduces McKinley to Pauline York as the "23rd President of the United States". McKinley corrects him by saying "25th" (which is correct). However, McKinley's lips say "24th" (which is incorrect as Grover Cleveland was the 24th) and the "25th" is a voice over (though it's difficult to make out as McKinley, played by Lloyd Corrigan, is doffing his hat, partially obscuring his mouth as he speaks).
At the beginning of the scene just after the investigator trips on a roller skate on the stairs, there is a blackboard listing crime clues. The shadow of the boom microphone is seen moving down the left side of the blackboard. This appears on the Turner Classic Movie version and is several inches into the camera frame.
At the magazine staff meeting right after an investigator has visited Louise Patterson and gotten the name of "the blonde woman", the shadow of the boom microphone can be seen moving on the upper left of the chalkboard frame.
Killer Earl Janoth (Charles Laughton) dispatches his employee Steve Hagen (George Macready) to the crime scene to eliminate any evidence connecting him to victim Pauline York (Rita Johnson). Hagen alters the broken clock time as well as removing the murder weapon and miscellaneous incriminating evidence. What Hagen failed to notice and left behind is the potentially incriminating evidence of a photo of the real killer, Earl Janoth, prominently displayed in the apartment.