Revealing mistake: In episode 1, watch out for the wires pulling K-9 along the beach. (Unlike a real dog, K-9 didn't take well to the soft sandy surface and sank into the sand. Pulling K-9 along on wires provided an illusion of movement when the motor inside wasn't up to the job).
Continuity mistake: Due to some sloppy editing of the "cliffhanger ending" at the end of episode 2, and again at the start of episode 3, the Doctor, Mel and Burton are seen to freeze in place for no apparent reason, just BEFORE we see the Bannermen line up and aim their guns at them.
Factual error: When The Doctor questions the arrested Chinaman in the Police Station (in episode one), the Doctor claims to be able to speak several Chinese dialects. What the Doctor actually says to the Chinaman is "How Are You?" in Cantonese, but the rest of what the Doctor says is completely improvised gibberish that SOUNDS Chinese, but is, in fact, complete nonsense.
Factual error: The Naval base that features in "The Curse of Fenric" is supposed to be on the coast of North East England. (A line of dialogue in the story refers to Maidens Bay - two miles from the base - as being in "Northumberland". Yet all the maps and charts on the walls are of the coast of South-East England (Sussex and Kent in particular) which is 250 miles from where the story is supposed to be set.
Plot hole: At the end of episode 1, the 'cliffhanger' ending is that the Doctor climbs over some railings, and then, by looping his umbrella on the railing, he begins to climb down. He looks down and it seems to be a very long way down. This is resolved in episode 2 by having Glitz walk around and let the Doctor step on his shoulders. Not only is it silly, but because Glitz walked around, it also makes the previous 'cliffhanger ending' totally meaningless.
Audio problem: In episode 3 of this story it is noticeable that practically every sound in this episode has been re-dubbed and as a result is out of synch - sometimes quite badly. An example is when Milton Johns has his thugs break into Giles Kent's trailer - there's a three-second lag between the breaking of the plates and the sounds they make. (This may be simply due to the only surviving copies in the BBC Archives being these "badly dubbed" ones, recovered from obscure sources).
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).