Visible crew/equipment: In episode four, when the Doctor is congratulating himself on blowing up the Swarm, the shadow of a boom mike can be seen.
Visible crew/equipment: In the first scene of episode two, as the Doctor collapses on the ground, a person can be seen moving in the back of the set.
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.
The Doctor: You can't rule the world in hiding. You've got to come out on the balcony sometimes and wave a tentacle, if you pardon the expression.
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