Plot hole: The linchpin of the plot makes no sense whatsoever. In an effort to diffuse international tension, the superpowers would allow Britain to publish the codes that would allow anyone in the world to launch their nuclear missiles? And then, after going through the drama of not one but two countdowns, it's revealed that the superpowers can just activate safety over-rides to prevent the launch. So how in Hades did the SRS have any threat whatsoever to wield over them and issue their demands?

Revealing: Perhaps criticizing the special effects is unfair, but the extremely obvious use of toy tanks and a rag-doll Sarah in certain scenes is one of the worst examples in the history of the show.

Deliberate "mistake": Benton is a Warrant Officer in this story, but the end credits refer to him as "Sergeant Benton".

Audio problem: When the Doctor chops a brick in half, you can tell that the brick is really made from balsa wood by listening to the sound it makes when it hits the ground.

Other: When the K1 Robot breaks out of Kettlewells factory in part 2 or 3, it trips up.

Plot hole: The SRS has, at their command, a robot who can punch through armor plate and dig through solid rock. So why do they go to such great lengths to get the disintegrator gun - only to use it to open a safe the robot could easily have ripped open?

Continuity: At one point in the story, the Doctor hurriedly types out a letter to leave for the Brigadier and Sarah-Jane, and pins it to the TARDIS. Later when Sarah-Jane reads the note, you can see it's been handwritten.

Deliberate "mistake": In episode three, the Doctor and Vira meet a half-changed Noah. He holds a gun on them, then drops it at Vira's feet and the door is suddenly closed. This is because the scene was originally longer and involved Noah begging Vira to kill him before the Doctor shuts the door. it was trimmed down to get it past BBC censors.

Revealing: When Vira emerge from the hibernation chamber, the Styrofoam that forms the pallet squeaks.

Continuity: When the Doctor and Harry arrive in the cryogenic chamber, they walk around for some time inspecting the chamber and talking. Towards the end, they notice a green slime trail newly appeared on the floor. But the trail was visible in the very first overhead shot of the chamber.

Revealing: As the cast move through the transom of the space station, observe the starscape through the lower windows. At certain points you can see a thin strip of light where the black curtain used for the stellar backdrop meets the studio floor.

Continuity: At the very beginning, Sarah comments to the Doctor that they haven't returned to "the beacon". But when they were last on Nerva - in Ark in Space - it was referred to as a space station. It doesn't become Nerva beacon until the next episode.

Continuity: When the Doctor drops the time ring, it lands very close to the wall. When he picks it up later, however, it's in the middle of the corridor.

Revealing: During the Doctor's early conversation with the timelord, there is fog swirling around them. But when the Doctor agrees to take on the mission, it suddenly appears very concentrated around the tip of the rocks behind them, as if generated from a source behind the rocks. Normal fog would not do this, and it's clear that dry ice was used as the fog effect (DVD commentary confirms this).

Continuity: When Sarah falls from the scaffolding, a plank of wood saves her from death. Trouble is, she fell a foot from the outside of the scaffolding and most of the plank was inside the scaffolding, therefore her feet would have hit it and she could only have landed with her head pointing inwards. But when we see her, her head is pointing outwards.

Visible crew/equipment: Just after the Daleks move past the Doctor and the female Thal soldier in the corridor, a piece of lighting equipment is reflected on the wall behind her.

Continuity: When the Doctor, Harry and Sarah are viewing the meeting on the monitor, a scientist suddenly appears to Gharman's left when Davros starts to speak.

Revealing: When Davros moves around in his chair, look closely at his stomach and you can see his body rotating. This is the actor pushing the chair with his feet, but Davros is supposed to be paralysed below the waist.

Revealing: In episode 6, when the Thals detonate the charges, smoke is seen coming through the door before the detonator is pressed down.

Continuity: In episode 6, when the lead Dalek is saying 'Pity? I have no understanding of the word' the lights on the top of its dome stop working. They start working again in the following shot.