Continuity mistake: 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.
Continuity mistake: 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.
Continuity mistake: 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 mistake: The use of gold against the Cybermen is wildly inconsistent in this episode. It is said that it suffocates them. Yet Cyberman have been seen functioning perfectly in vacuum (The Moonbase and the Wheel in Space), it affects the Cybermats, who are little more than metal drones, and it affects the Cybermen's radar, which does not breath.