Visible crew/equipment: Episode 1: After Sarah teases the Doctor a bit when he's being somewhat moody, just as the TARDIS is suddenly thrust off its flight path (which was back to UNIT), Sarah and the Doctor are thrown about in the wonky console room (because the camera turns sideways), and the second camera becomes visible at the left side of the screen.
Revealing mistake: (Part 4) After Omega takes off his mask and looks in the mirror, when he sees that his physical body doesn't exist he puts his mask back on and screams, but when he shouts, "If I exist only by my will, then my will is to destroy," Omega's mask pops up off his face, and we can see the actor's blackened face as he shouts the last bit of his dialogue. (00:07:10)
Plot hole: The Brigadier says UNIT HQ is 'a Top Secret establishment'. It is, in fact, so secret that it has a large sign outside informing the world not only of its function, but also the name of the commanding officer.
The Daleks - S1-E2
Visible crew/equipment: About 90 seconds into Episode Two, the shadow of the boom mic crosses Ian's face. (00:01:29)
Visible crew/equipment: (Ep. 2) When The Doctor, Jo, and Dr. Tyler are taken into Omega's palace, in the first shot of the multiple passageways (right before Jo says that it looks like Aladdin's cave) the curved chalk actor's mark is visible on the floor, then we see it again in three following shots. Note that when they're all escorted further inside, just as Dr. Tyler comments about the matter/anti-matter not making sense, Jon Pertwee looks down, presumably to make sure he hits his mark.
Plot hole: After UNIT HQ disappears, you can see a patch of grass where the UNIT HQ used to be...but just exactly how can grass grow underneath a building?
Earthshock - S19-E6
Revealing mistake: When the Cybermen break through to the bridge, they brush against the door and it bends as if made of cardboard. It was supposed to be made of reinforced steel. (01:09:10)
Other mistake: The Cyber Leader is supposed to follow the Master's footsteps exactly in order to get across the chessboard. He doesn't. This cannot be classed as a character mistake because the Cyber Leader knew what was involved. I believe this mistake was caused by the actor not being able to see the chessboard through his helmet.
Destiny of the Daleks - S17-E1
Revealing mistake: Right after Davros yells "The Daleks will rule the universe," in Episode 3, one of the Dalek operators accidentally starts to lift the top off his Dalek before the scene ends. (00:17:35)
Earthshock - S19-E6
Continuity mistake: The hole that the Cybermen make in the door increases in size just before they step through it. (01:09:10)
Revealing mistake: When a Cyberman's arm is sliced off by the Robot's shuriken, you can see his real arm bulging out at the right side of his body. (01:00:25)
Destiny of the Daleks - S17-E1
Revealing mistake: When the Doctor and his companion are running past explosions in the sand, you can see the scrape marks from where the effects team covered up the holes containing the charges. (00:13:00)
Earthshock - S19-E6
Factual error: As the spaceship approaches the Earth of 65 million years ago, the Earth looks very much like the present day one. Hasn't anyone heard of "continental drift"? The Earth's land masses of 65 million years ago were not where they are now. (This has been fixed on the DVD)
The Caves of Androzani - S21-E6
Revealing mistake: When Stotz cuts a hole in his ship's door, you can see that the inside of it is made of white polystyrene. They painted the outside of it but obviously forgot about the inside. Also listen to the sound it makes when Stotz punches the hole through - it sounds exactly like polystyrene.
Continuity mistake: In episode 4, the Doctor's bow shrinks from when he talks to Liz Shaw while in prison, to when he stows aboard a jeep later. In the DVD commentary, producer Barry Letts blames himself for this mistake. (00:13:00 - 00:19:50)
Revealing mistake: When the Doctor and Jo see Bessie in the middle of the rocky area, there is the white chalky substance from the ground on her wheels, indicating she was driven to the spot rather than being "transported" to the black-hole.
Genesis of the Daleks - S12-E4
Continuity mistake: The Doctor's big brown overcoat was taken by the regular Kaled army in episode one and disappeared. In episode six, the Doctor goes into the incubator room and comes out with his brown overcoat magically restored.
Continuity mistake: In episode 1, when the Doctor opens the garage doors with his 'door handle', note the two men in the background, the position of the car behind him and the white pillar. In the next shot, the men and pillar disappear and the car moves to a different spot. In the shot after that, the tanker truck is now parked in front of a pillar. You can see near the beginning of episode 3 that there are two entirely different buildings that the tanker had been filmed in front of. (00:14:25)
Attack of the Cybermen - S22-E1
Revealing mistake: When Lytton stabs the Cybercontroller, some of the Cybercontroller's 'blood' gets splashed on the camera. (The 'blood' is green-dyed water spraying from the Cybercontroller's arm.).
Revealing mistake: In episode 3, when the second Doctor, the Brigadier and Benton emerge from the TARDIS, the doors are left completely open with the camera looking straight into them for a long time, revealing not only the emptiness of the police box shell, but also that there's a pair of secret doors on the opposite side to the entrance doors. These 'secret doors' come in handy during episode 4, allowing seven people to come out of the police box in the final confrontation with Omega. (Well, it is stated time and again that the TARDIS is "bigger on the inside than the outside". Now we know how that happens...).
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