Visible crew/equipment: After leaving her husband at train station, Harriet takes a cab to meet a man at a cabin. After a few mins, she takes the man's car to go home. As she drives away, there's a reflection of a man, in suit with glasses (possibly the director) standing offscreen. (00:05:20)Keiferct
Other mistake: In an abandoned house, a 14-year-old boy finds a satchel filled with money taken in a holdup at a finance company. He uses $50 to buy a robe for his grandfather, then hides the rest of the money in his grandfather's house. During the trial of the grandfather, who is accused of being one of the holdup men, the police lieutenant says, "We found all the money except for $12,000. We conducted a second search of the home and found the rest of it." He should have noted that $50 was missing.
Plot hole: Claire Allison spends the night in an upstairs bedroom in the Jennings' home. In the middle of the night, she opens a walk-in closet to get another blanket and finds a small printing press. Later in the day, Claire returns to the home, accompanied by Perry Mason, who tells Mr. Jennings that Claire has received threatening letters from Mrs. Jennings' former husband, who still has a key to the Jennings' home, and the printing press may have been used to address the envelopes. Mr. Jennings says, "We'd better investigate" and immediately goes upstairs and opens the closet, even though neither Claire nor Perry Mason said where Claire saw the printing press.