The Case of the Red-Faced Thespian - S4-E12
Revealing mistake: In the climax of the episode, Magnum tackles Higgins to the ground - but it's an obvious stunt double. (00:45:10)
The Case of the Red-Faced Thespian - S4-E12
Plot hole: Magnum mentions that he spoke "one hour ago" to Robin, getting the description of Lowell Xavier Jameson. All the suspects are at the estate, and Tanaka arrived, apparently with an arrest warrant for Higgins, when Magnum still hadn't spoken with Robin. Magnum did have to bargain for Tanaka to put off the operations by two hours; it does not make sense that he'd be around at the estate one 1 full hour more for unexplained reasons.
The Case of the Red-Faced Thespian - S4-E12
Other mistake: As said in the ending, Lady Cynthia Warrington-Stout was on Higgins' bed when she was found; the room looks nothing like Higgins' room as shown in Foiled Again (3-8).
The Case of the Red-Faced Thespian - S4-E12
Plot hole: Magnum is incredibly slow to unveil the charade at the base of the episode; he does it based on something marginal (Robin Masters does not do "research" for his novels), but he completely overlooks the key plot point of 1-17 "J. Digger Doyle" was that he does not use a typewriter, but he dictates the novels.
The Case of the Red-Faced Thespian - S4-E12
Plot hole: The plot of the episode has a big reveal that tricks the viewer making zero sense; the victim is alive and well, but there's an ambulance, they bring the victim away, and Magnum spends most of the episode in the noble effort of saving Higgins from a murder accusation. Magnum mentions "I figured I still had about a half an hour before Tanaka returned with a warrant for Higgins' arrest." even in his internal monologue, so it's not that he was trying to hide the fact from other characters.
