Revealing mistake: When the crack in the wall breaks open, the wall wobbles rather obviously.
Visible crew/equipment: As the Doctor is watching Will emerge from the priest-hole, look over his shoulder at the far corner. Someone or something appears to be moving in the shadows there.
Other mistake: What happens to Tegan's bag when the plague victim transforms into the cavalier? It doesn't appear to be left behind, even when the cavalier later vanishes.
The Doctor: This is a situation that requires tact and finesse. Fortunately, I am blessed with both.
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: If The Master had closed the CVE it would have resulted in the total destruction of the universe. Would he have really closed the CVE if the peoples of the universe hadn't responded to him? Isn't that the complete opposite of what The Master wants, i.e. complete control of the universe? Also, if the universe had been destroyed, who and what would he have left to control?
Chosen answer: The Master is insane. If he can't control the universe, he'd rather see it destroyed.
Captain Defenestrator