When John Robinson asks the robot if he and Don will be able to return to their planet, the robots answer is a take off on Isaac Asimov's first law of robotics: A robot may not injure humans or through inaction cause harm to human beings.
At one point in "Hades", Smith bumps into a rather rigid fire-snorting Chinese dragon. The same dragon, this time as a statue amid the other strange incoherent artifacts in another interdeminsional world, was seen in the previous season's "Magic Mirror" (16 February 1966).
Smith encounters a green monster with goggle shaped eyes and an exposed double-dome brain. This creature is based on the famous Metaluna Mutant from 'This Island Earth' 1955.
This is the exact mid-point in the original series' run. It is the 42nd episode with another 42 to come.
This is the first truly Marta Kristen centric episode