Deliberate mistake: The huge system and missile control boards containing the failsafe units suddenly appear on Seaview's bulkheads just for this episode. And of course, they disappear after this, never to be seen again. (00:15:00)
Continuity mistake: Captain Crane removes a failsafe key, which is hanging outside his uniform collar, from around his neck. It wasn't there in the previous shot. (00:10:30)
