Deliberate mistake: In episode 3 of 'The Web Planet' (titled 'Escape To Danger') watch as one of the Zarbi operators, his vision impaired by the costume, runs right into the camera. So far behind schedule was the recording of this episode, however, that this mistake was retained in the finished episode, as there was no time left for a re-take.
Doctor Who (1963)
1 deliberate mistake in The Web Planet - chronological order
Revealing mistake: (Ep. 4) After Omega takes off his mask and looks in the mirror, when he sees that his physical body doesn't exist he puts his mask back on and screams, but when he shouts, "If I exist only by my will, then my will is to destroy," Omega's mask pops up off his face, and we can see the actor's blackened face as he shouts the last bit of his dialogue. (00:07:10)
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: 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