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)
Audio problem: In episode 3, when the Doctor is squaring off against the drooling madman, it's clear in most of the shots that the sound of the madman's panting is not reflected in his facial and body movements. (00:10:20)
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)
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)
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)
Plot hole: Just after the Doctor meets the Brigade Leader, he looks at the countdown and says in a very loud voice, "only 3 hours and 22 minutes". Several minutes later, when he leaves the office after stunning Benton, the countdown still reads 3 hours and 22 minutes. Did the countdown clock stop? (00:15:10 - 00:22:10)
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)
Revealing mistake: In episode 6, during the generally poor fight sequence between the Brigade Leader and Greg Sutton, Greg's first punch to the face misses the Brigade Leader by miles. (00:19:10)
Visible crew/equipment: In episode two, the shadow of the boom mic crosses Jon Pertwee's head as he's giving the "epsilon coordinates" to Liz.
Revealing mistake: In episode one, a wall wobbles as Benton hammers a nail into it.
Visible crew/equipment: When Liz and the Brigadier are arguing toward the end, a shadow moves in the background. It's in the wrong place and too soon to be the Doctor returning.
Other mistake: The power output in the Doctor's shed reads "Megga Volts" instead of "megavolts."
Continuity mistake: At the beginning of the third episode, the countdown reads 49:18:32 in the close up, but when the Brigadier walks across the room it now is on 49:15:01. (In other words three minutes thirty one seconds have elapsed). Yet these two events are about ten seconds apart. Also, the timer should never read either of these times. The professor moved the countdown forward to exactly 49 hours (49:00:00).
Continuity mistake: Throughout the serial, Bessie's tires are dry in studio shots (inside the Doctor's shed) and wet on location shots.
Continuity mistake: Liz's hairstyle is different in indoor and outdoor shots.
Revealing mistake: When Stahlman is killed - the second time - the shot of his green primord face reveals the actor's unpainted neck under his beard.
Character mistake: When the Brigade Leader shoots the Primord, he noticeably closes his eye. He is supposed to be a high ranking well trained officer.
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