The Case of the Wandering Widow - S4-E6
Character mistake: As Mr. Morgan (Dean Harens) is being examined by the prosecutor he's also sitting in the gallery.
Perry Mason (1957)
1 character mistake in season 4 - chronological order
Starring: Raymond Burr, Barbara Hale, Ray Collins, William Hopper
The Case of the Sausalito Sunrise - S9-E19
Other mistake: The owner of a Los Angeles trucking company tells a driver who is going to San Francisco, "Take the San Diego Freeway toward Bakersfield." That freeway runs from San Diego to San Fernando. It does not go toward Bakersfield.
Hamilton Burger: Incompetent, irrelevant, and immaterial.