The Night of the Bubbling Death - S3-E1
Revealing mistake: As West drags the body of a bad guy out of the scene, the supposedly unconscious man is holding his head up so as not to have his head dragged through the dirt. A truly unconscious person's head would flop back on to the ground.
There Were Giants in the Earth - S1-E5
Revealing mistake: The Robot's lights wink out for a few seconds as it disables the force field.