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.
Revealing mistake: When building the shelter, it takes both John and Don to lift the supposedly heavy panels; however when upright the panels bend easily when gripped. Obviously some sort of styrofoam/aluminium foil prop.
Revealing mistake: At the beginning of the episode, Dr Smith is painting a painting. The commander picks it up and questions it. Smith grabs it back and puts it back on the easel but puts it in a different direction but thinks nothing of it.
