Continuity mistake: In "The Ribos Operation", Romana stated that her age was 'nearly 140' (presumably Earth years, although this is never confirmed). However, in the later story "City of Death", Romana says that she is '125 years old'.
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).
Delta and the Bannermen - S24-E3
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.
Continuity mistake: The Doctor has his scarf when he first meets with the Vardans. When he returns to the TARDIS, it is on the coat rack.
Planet of the Spiders - S11-E5
Revealing mistake: In episode one, when a psychic wind is blowing everything all over the Doctor's lab, look for the hatstand standing steady as a rock.
The Talons of Weng-Chiang - S14-E6
Visible crew/equipment: The shadow of a boom mike is visible on the curtains near the stage in the final fight.
Genesis of the Daleks - S12-E4
Visible crew/equipment: In episode 1, when General Ravon is being held at gunpoint and he says 'they won't last much longer', the shadow of the boom pole is moving on the door behind him. (00:15:15)
Planet of the Spiders - S11-E5
Revealing mistake: A bad CSO match up occurs in episode one, when Clegg is making the breakfast tray float across the room. The Brigadier is looking at a point some distance from the CSO'd tray.
Continuity mistake: In episode four, when the Doctor and Ian return to Frankenstein's lab, the monster is lying on the table in the opposite direction from when they left.
Continuity mistake: In episode five, the Doctor escapes from Global during the day. When Yates is caught it's dark, but the next scene, on the slag heap, is in daylight again.
Destiny of the Daleks - S17-E1
Revealing mistake: In episode one, when the Doctor and his companion hear rumbling and the ground moves beneath their feet, when they step away you can see that they are standing on different ground than before, and it's not shaking. (00:08:00)
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.
Delta and the Bannermen - S24-E3
Plot hole: In episode 3, the swarm of "bees" attacking Gavrok and his men look nothing like real bees, being far too large when they approach the camera's position. (In fact, stock footage of a swarm of locusts was used.).
Audio problem: During Hartnell's big speech about the creation of a solar system, a cough can be heard from off-stage.
Plot hole: At the very end, the TARDIS materializes in Antarctica and Sarah comments that they've returned. But the TARDIS was never there - the Doctor and Sarah went to Antarctica by helicopter.
Suggested correction: But THEY were there.
Visible crew/equipment: The boom mike can be seen in the first scene set at the stones in the upper right corner of the screen.
Revenge of the Cybermen - S12-E5
Continuity mistake: At the end of "Genesis of the Daleks", the Doctor was wearing his big brown overcoat. However, at the beginning of this episode, he is wearing his red jacket again.
Dragonfire - S24-E4
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.
The Enemy of the World - S5-E4
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).