Plot hole: Yartek tried to disguise himself as Arbitan by wearing Arbitan's robes and pulling the hood over his giant rubber head. Strangely enough, no-one seems to notice that "Arbitan" is now suddenly speaking with a completely different voice, and now has a two-foot-tall-head.
Revealing mistake: In episode one, the Voord that falls down the pyramid shaft is very obviously a cardboard cutout.
Visible crew/equipment: In episode one, when the Doctor is near the pyramid, Ian's leg can be seen just before the shot ends, as the consecutive shots used the same section of wall.
Continuity mistake: In episode one, Barbara activates her travel dial only seconds before the others. But in episode two, she's been at Morphoton for quite some time - long enough to change clothes, pick fabrics and get food.
Visible crew/equipment: In episode one, when Ian is first seen inside the pyramid, someone stumbles through the background. Some have claimed this is Barbara, but it happens too fast to tell.
Visible crew/equipment: In episode one, when a Voord falls through a pyramid door, a stage hand can be briefly seen on the other side.
Plot hole: In episode 4, we're informed that Susan and Sabetha traded their travel dials for firewood, and Ian trades his for furs. They do not take them with them into the caves later on. This only appears to make sense from an out-of-universe perspective: Terry Nation needed the characters to part with their travel dials, so they couldn't have just escaped after collecting the key.
Revealing mistake: When Barbara first uses her wrist transporter, Ian steps towards her. As his hand enters the black half of the screen, it disappears.
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