Revealing mistake: In episode four, the camera wobbles when the Doctor brings the gun to Stael.
Revealing mistake: When Vira emerges from the hibernation chamber, the Styrofoam that forms the pallet squeaks.
Revealing mistake: There are times in this episode where the strings holding up the Captain's trophies are visible. Additionally, the "billions of tons" of planet sways back and forth slightly.
Revealing mistake: The queen, suspended in the last few moments of life, can be seen to breathe rapidly and blink.
The Creature from the Pit - S17-E3
Revealing mistake: Torvin, stunned by K9, turns before falling, as if looking for somewhere comfortable to land; and the guard who goes to attack K9 and Romana acts very strangely: he stops and curls up before K9's blast hits him, almost as if he knows what is going to happen next.
Revealing mistake: When the Morbius creature falls over the cliff, he bumps into the camera.
Revealing mistake: Jo is hit by a rather large rock. Katy Manning visibly winces before it strikes (and bounces off lightly).
The Invasion - S6-E3
Revealing mistake: The Cyberman that falls from the roof in the final episode is obviously an empty costume. (This is another example of a mistake being repeated from an earlier episode. See the earlier story "The Tomb of the Cybermen" for a note about the same 'revealing' mistake happening).
The Dalek Invasion of Earth - S2-E2
Revealing mistake: When Barbara, Jenny and Dortman are running through the deserted streets of London, in the very last shot, if you look to the left of the screen, you can see some people walking down the street.
Revealing mistake: When SV7 comes into Taren Capel's workshop, its usual eyes are missing - only black discs are visible (later in the scene, these are overlaid with the red speckled pattern to show the robot being reprogrammed).
Revealing mistake: The "American" accents in this episode are all over the map and frequently inconsistent within the same scene.
Revealing mistake: In episode 2, when Scaroth locks the Doctor, Romana and Duggan in the basement dungeon, and Duggan lights the lamp, notice that Tom Baker is standing between that lamp and the actual stage light used to brighten the scene. So, as the lamp is lit in front of him, Tom Baker's BACK (and not his front) glows bright under the increased illumination.
Revealing mistake: When Taren Capel is modifying a Voc robot, there is a shot of the robot's agitated hands as he reassures it. On the robot's silver gloves, the "Marigold" logo is visible. ('Marigold' make rubber gloves that are used when washing dishes). Perhaps that particular Voc robot had been assigned to dish-washing duty that day...
Revealing mistake: The shot of the two motorbikes colliding with each other and exploding into flames is very unconvincing. Watch carefully, and you can "see the join" where a shot of the two motorbikes passing each other (and NOT colliding) is replaced by a "staged" fiery explosion.
Revealing mistake: The bottle of acid that the Doctor throws at Mestor breaks open well before it hits the shield that Mestor is projecting around himself.
The Tomb of the Cybermen - S5-E1
Revealing mistake: When Jamie and the Doctor try to open the doors to the tomb, watch them push it with their feet.
Genesis of the Daleks - S12-E4
Revealing mistake: During the Doctor's early conversation with the Time Lord, there is fog swirling around them. 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). (00:02:45)
Revealing mistake: The knife that Leela kills Meeker with is very obviously loosely attached to his clothes. It wobbles back and forth after he falls.
The Caves of Androzani - S21-E6
Revealing mistake: In the very first shot of Morgus' hand-held computer, it's obvious it's a TV remote. Look for the controls for contrast and tint.
The Daemons - S8-E5
Revealing mistake: When the Brigadier crosses over to the RAF and the plane, you can see the plane is a model with a stick to control it just under the nose.
Answer: It was never destroyed on-screen; it was intact at the end of the TV movie, and destroyed by the start of the 2005 series. It was destroyed in the novel "The Ancestor Cell," but in a completely different manner to what happened in the series.
DaveJB