Revealing mistake: In episode 6, when the Brigade Leader shoots the Primord Stahlmann, he falls down but no bullet holes are made in his costume. Thanks to Producer Barry Letts for spotting this one on the DVD commentary. (00:17:00)
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.
The Dalek Invasion of Earth - S2-E2
Revealing mistake: In episode 1, one of the windows on the TARDIS is leaning in, but by the time we get to episode 6 it has managed to fix itself. Also when the bridge falls down you can see the poster through the gap, showing that there is no back to the TARDIS.
Revealing mistake: During episode three, in the mining area, just before the city's root appears, the tracks on which The Daleks move can be seen.
Revealing mistake: Perhaps criticizing the special effects is unfair, but the extremely obvious use of toy tanks and a rag-doll Sarah in certain scenes is one of the worst examples in the history of the show.
The Monster of Peladon - S11-E4
Revealing mistake: When the Doctor and Sarah arrives in episode one, they leave the doors of the TARDIS open and the empty inside of the TARDIS can be seen.
The Daleks - S1-E2
Revealing mistake: The Dalek that Ian hides from in Episode Seven has a bad castor. The flopping of one of its wheels is audible. (00:08:20)
Revealing mistake: When the Doctor and Leela are put in Uvanov's cabin to await questioning there is a shot of a robot listening to them in which you can see the actor's neck.
Revealing mistake: K-9 has a bit of difficulty in the mansion in episode two. The thread pulling him is visible and Baker has to help him over a flagstone.
Revealing mistake: The TARDIS manual Romana shows the Doctor quite obviously has a couple of hand-written pages jammed in so that they can be easily ripped out.
Revealing mistake: In episode 5, when the Doctor kills the Primord with the fire extinguisher, he says 'he's dead this time' but the Primord is breathing. (00:07:10)
Revealing mistake: In episode 3, when the Doctor is squaring off against the drooling madman, the madman is inside the perimeter railings. When the soldiers shoot him and he 'falls', however, he has moved outside the railings, enabling the stuntman to jump safely. (00:10:40)
The Caves of Androzani - S21-E6
Revealing mistake: When the Doctor is looking for a way to break through his manacles, he brushes against the machine with the bright light that he eventually uses to cut them. When he does, it wobbles when it obviously should remain motionless.
Revealing mistake: When Fenric (having possessed the body of Judson) knocks over the chess pieces on the board, watch one piece (The King) in particular. You can see a 'hinge' made of white sticky tape attached to the board and the base of the chess piece, presumably there to stop the chess piece rolling away onto the floor.
The Trial of a Time Lord 9-12 (aka Terror of the Vervoids) - S23-E3
Revealing mistake: In episode 12, while the Vervoids are dying, look out for the ones wearing sneakers and tracksuit pants (complete with drawstrings)...
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 Caves of Androzani - S21-E6
Revealing mistake: It's already been mentioned that Peri bounces off a mattress, but watch what happens when the Doctor walks over to see if she's all right. The 'cave' floor visibly moves under his foot.
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