The Case of the Wayward Wife - S3-E13
Revealing mistake: Perry and Della step into an elevator but there is no line in the floor between the floor they are on and the elevator floor. It's continuous.
Perry Mason (1957)
1 revealing mistake in season 3 - 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.