Factual error: Titan is depicted as a Moon-like landscape with no atmosphere. In actual fact, Titan has a nitrogen/argon atmosphere denser than Earth's, and a reddish-orange sky. This was detected almost thirty years before "The Invisible Enemy" was made, so somebody slipped up in doing their research.
Underworld - S15-E5
Factual error: While it's true the centre of a planet would have no net gravity, the weightless condition would be everywhere, not concentrated in a convenient shaft.
Answer: In 'The Five Doctors', three separate Cyberleaders are definitely used. So it's likely that Cyberleaders are like unit commanders, of which a fair-sized army might have several.
Daria Sigma