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.
Continuity mistake: At one point in the story, the Doctor hurriedly types out a letter to leave for the Brigadier and Sarah Jane, and pins it to the TARDIS. Later when Sarah Jane reads the note, you can see it's been handwritten.
Plot hole: The linchpin of the plot makes no sense whatsoever. In an effort to diffuse international tension, the superpowers would allow Britain to publish the codes that would allow anyone in the world to launch their nuclear missiles? And then, after going through the drama of not one but two countdowns, it's revealed that the superpowers can just activate safety over-rides to prevent the launch. So how in Hades did the SRS have any threat whatsoever to wield over them and issue their demands?
Audio problem: When the Doctor chops a brick in half, you can tell that the brick is really made from balsa wood by listening to the sound it makes when it hits the ground.
Other mistake: When the K1 Robot breaks out of Kettlewells factory in part 2 or 3, it trips up.
Deliberate mistake: Benton is a Warrant Officer in this story, but the end credits refer to him as "Sergeant Benton".
Revealing mistake: In the last part, the giant robot steps on a soldier. There are several things wrong with this scene. 1) You can see the matte halo around the robot's foot. 2) The robot does not leave a footprint. And 3), after the soldier gets squashed, his arm is visible waving over the robot's 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