When Ellery figures out why Manners wasn't in the elevator as it reached the 12th floor, he rushes off to the murder scene to check his theory. His father follows along with him in such haste that he leaves the refrigerator door open and the lights on in their front hall. But when two return home later, the lights are all off.
When Ellery visits the newspaper building he examines the elevator at the 5th floor. He presses the button for the 12th floor, but the elevator in the shaft is seen going down instead of up. Ellery then end up in the lobby.
When Ellery goes to see Thornton Johns in the City Room the door Ellery come through is marked 1063 which would indicate that the City Room is on the 10th floor, but directly across the hall is a door marked 1262 which could only be on the 12th floor. No skyscraper in the city would have mismatched doors in hallways.
When Ellery is talking to Harriet Manners he addresses her as "Mrs. Manners". It is already established that Harriet is the Sister of the victim Henry Manners. If Harriet was married, she would have taken her husband's name as was the custom in 1947. If she was divorced and using her last name she would not bother with the 'Mrs' title.