Visible crew/equipment: Episode 5, while Zoe and Jamie are at the Weather Control Station, when they hear the Ice Warrior approaching they both hide, and the actor's T-mark is visible on the floor. (00:18:00)
Revealing mistake: When Slaar discovers the video link is operational and being viewed on Earth, in the closeups of Slaar we can see the edge of the reptilian mask and the actor's real human neck. (00:20:40)
Revealing mistake: Episode 6, while in the barn Von Weich asks Moor (played by David Troughton, son of Patrick, who plays The Doctor) to retrieve the monocle in his pocket. Then, when Von Weich continues with his plan, in his closeups we can see a vertical piece of mesh tape over the actor's right eye. (00:13:05)
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