I of Newton - S1-E29
Revealing mistake: Set lights are visible in the Demon's sunglasses while he is wearing them and when he is not.
Tooth and Consequences - S1-E41
Revealing mistake: The glue holding the mustache to actor David Birney's (Dr. Myron Mandel) upper lip is visible.
Revealing mistake: In the close-up of Frost's hand while the doctor is stitching up his wound, you can see the delineated edges of the latex make-up patch used to simulate the injury. (00:12:30)
Crazy as a Soup Sandwich - S3-E29
Revealing mistake: The Styrofoam head Gus tears in half shows it has been pre-cut, making it easier for him to rip in half.
Crazy as a Soup Sandwich - S3-E29
Revealing mistake: The supposedly heavy cardboard boxes Gus, Bork and Arky pile against the door are very light and obviously empty.
Answer: He was in an accident.