Visible crew/equipment: A stagehand's foot is visible before the party beams down. (00:09:10)
Other mistake: In the cells, the particular strip of metal Kirk pulls loose has rivets that are noticeably different than the rest, making it easier to pull loose.
Revealing mistake: When Kirk holds a golden egg, a seam can be seen on it. (00:43:55)
Answer: After causing Norman to overload, all of the other androids shut down. The same could be said for the androids on the Enterprise.