Factual Error: Barclay claims the Z-bomb might turn Mondas into a sun or supernova. That isn't possible.
Revealing Mistake: When Cutler is talking about getting permission to use the Z-bomb, the camera bumps against something.
Credit Error: Gerry Davis is listed in the credits as Gerry Davies.
Obviously, the writers haven't decided yet how many hearts the Doctor has. Polly checks his pulse and finds it normal.
When the Cybermen are ambushed outside, one of them has part of his headpiece(one of the "jug handles") come loose.
The dialogue between Cutler and the technician at that beginning of the episode is slightly different from that which closed Episode 2.
Although credited, William Hartnell does not appear in this episode. The script was rewritten by Gerry Davis to accommodate the actor's absence through illness. The Doctor, who is seen only from behind, is played by Gordon Craig.