The Case of the Cheating Chancellor - S9-E4
Character mistake: Paul Drake tells Perry the "speedometer mileage" of a murdered man's automobile is 63,192. Speedometers do not show mileage. He meant to say "odometer."
Perry Mason (1957)
1 character mistake in season 9 - chronological order
Starring: Raymond Burr, Barbara Hale, Ray Collins, William Hopper
The Case of the Sausalito Sunrise - S9-E19
Revealing mistake: Secretary Della Street answers the telephone in Perry's office and speaks briefly with the owner of an art gallery. She presses the "hold" button, then hands the receiver to Perry and says, "It's Mr. Clune." Perry and Mr. Clune have a lengthy phone conversation, even though Della never released the "hold" button.
Hamilton Burger: Incompetent, irrelevant, and immaterial.