My Master, the Weakling - S3-E5
Other mistake: About 3 minutes into the show, Don Rickles goes around a corner to get some equipment but you can see his left arm at the corner while he's waiting for his spot to come back into the scene.
Other mistake: When Don and Dr Smith are stealing Sesmar's control unit, look closely at the base of the electronic arch - Raddion is hidden from sight but the careful eye will see his golden feet.
Other mistake: When the phone pole was installed with a "butt set" to talk with, they used it upside down for a long time. Someone must have brought it to the director, because they finally corrected the problem.