The Night of the Spanish Curse - S4-E14
Other mistake: Very close to the end of this episode during the fight scene, West and Gordon throw a guy directly into the camera - causing the shot to become unfocused and shaky.
The Wild Wild West (1965)
1 other mistake in The Night of the Spanish Curse - chronological order
Starring: Robert Conrad, Dick Cangey, Ross Martin, Whitey Hughes
The Night of the Avaricious Actuary - S4-E11
Continuity mistake: The letter from the Cyclops Insurance Company that Gordon pulls out of his pocket is folded, wrinkled, and well-worn. In the close up of the same letter West reads, the letter is white, pristine, and not wrinkled or folded.
The Night Dr. Loveless Died - S3-E4
Trivia: Dr. Loveless' uncle's name, "Liebknicht", is, aside from the addition of the letter "k", German for "Loveless".