Continuity: When Palmerdale wakes Harker up and asks him if he can use the telegraph, Harker answers him, then asks him to repeat the question, then answers again.

Plot hole: Palmerdale and the others hear someone on the stairs. Skinsale opens the door and, despite Reuben being right in front of him, looks downstairs and calls out to the Doctor before turning and suddenly spotting the lumbering keeper.

Revealing: At the very beginning of episode four, after Vince is killed, Reuben's body breathes and his fingers move.

Plot hole: The damage caused by the shuttle crashing into the Bi-Al Foundation is seen BEFORE it actually happens.

Visible crew/equipment: In episode four, when the Doctor is congratulating himself on blowing up the Swarm, the shadow of a boom mike can be seen.

Plot hole: The Doctor's clone is free of the virus - so why is Lowe's clone infected? Perhaps they infected him after cloning, in which case, why is Lowe cloned at all?

Continuity: Lowe's "frozen" makeup changes between when he emerges from the freezer and when Leela brings him a cup of coffee.

Factual error: Titan is depicted as a Moon-like landscape with no atmosphere. In actual fact, Titan has a nitrogen/argon atmosphere denser than Earth's, and a reddish-orange sky. This was detected almost thirty years before The Invisible Enemy was made, so somebody slipped up in doing their research.

Plot hole: Why doesn't Lowe simply shoot Leela the second her back is turned? Moreover, why does he take them to get medical help? It's made clear that the virus has everything it needs on Titan.

Revealing: In episode three, Leela collapses before K-9 shoots her, causing the CSO'd blast beam to miss her by a considerable margin.

Plot hole: After the Doctor and Leela's clones fade, the camera pans to show Leela's knife. But since the knife was created in the cloning process, it should have faded out with Leela, the Doctor and their other personal items.

Revealing: In episode 3, the section of wall K-9 knocks down is obviously precut, complete with debris on the floor.

Continuity: In episode three, the readout strip dangling from K-9's mouth shortens between when he shoots the infected crewman and shoot Leela.

Visible crew/equipment: In the first scene of episode two, as the Doctor collapses on the ground, a person can be seen moving in the back of the set.

Other: When Marius injects the cloned Doctor and Leela into the real Doctor, the syringe is full of air, although the later graphic shows them in liquid.

Revealing: The knife that Leela kills Meeker with is very obviously loosely attached to his clothes. It wobbles back and forth after he falls.

Revealing: When K-9 shoots one of the infected men, the blast beam appears to come out of his eyes, then moves down to his snout as the camera moves.

Continuity: The countdown clock in Marius' lab speeds up and slows down as needed for the plot.

Plot hole: The Nucleus of the Swarm is referred to on numerous occasions as being a virus. A virus is the simplest form of life yet recognised and is made up of bundles of DNA wrapped in a coating of proteins. They reproduce by attaching themselves to other lifeforms and releasing their genes into an unsuspecting host cell. Viruses do NOT lay eggs. Nor do they resemble prawns.(It would be more accurate to describe the Nucleus of the Swarm as a protozoan. These are simple animal lifeforms radically more advanced than viruses, around a fifth of which are parasitic although not all cause disease).

Other: Several shots are repeated during the implosion of the priory.