Plot hole: In episode 1, on both occasions that Silas P presents his card (first to Daphne S, then later to the Doctor), he gives it to his victims the wrong way around. They have to surreptitiously turn it over so that the side with his name on it is facing them, and then act surprised when he tells them to flip the card over to reveal the words "Happiness Patrol Undercover", as if they hadn't seen it before.
Doctor Who (1963)
1 plot hole in season 25 - 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)
Trivia: This Doctor Who story was originally scripted and produced as a four-episode story, but, just two weeks before transmission, upon viewing the story, co-creators Sydney Newman and Donald Wilson felt that the final two episodes (Episode 3, 'Crisis'; and Episode 4, 'The Urge to Live') should be combined into a single episode. The new 'condensed' episode incorporated the opening titles of 'Crisis' with the closing credits of 'The Urge to Live'.
Question: What caused the first Doctor to regenerate? Was it old age?
Chosen answer: No, it was the result of the Cybermen's draining Earth of energy to their world, that also affected the Doctor, draining his energy as well, causing him to regenerate.
Also In the episode it's said that his body was wearing thin.
Dan23