Plot hole: One sequence which makes little sense is when Ace triggers the booby-trap gas grenade on the chess board. Although the Doctor places a wastepaper basket over the grenade, the green toxic gas is very much present for the rest of the scene but does not harm either the Doctor or Ace, despite it being shown to have been lethal to the soldiers in a preceding scene. (NOTE: This error only applies the TV broadcast and the original video release; the error was noticed and corrected in 2003 for the DVD release of this Doctor Who story).
Doctor Who (1963)
61 plot holes - chronological order
Other mistake: The Brigadier says UNIT HQ is 'a Top Secret establishment'. It is, in fact, so secret that it has a large sign outside informing the world not only of its function, but also the name of the commanding officer.
Castrovalva - S19-E1
The Doctor: That's the trouble with regeneration, you never quite know what you're going to get. (00:12:15)
Planet of the Spiders - S11-E5
Trivia: The first thing the Third Doctor does on-screen is collapse out of the TARDIS, which is also the last thing he does in that incarnation.
Question: In which season and episode is Gallifrey destroyed, or is it just a shocking new plot development for the new series?
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