The Doctor is heard to stumble over the phrase, "I can't prove at this very moment", saying initially, "I can't improve at this very moment." Although William Hartnell was notorious for stumbling over dialogue (which, due to time and budget restrictions preventing retakes, was often left in the broadcasts), this isn't one of those occasions. According to the DVD trivia track, for some reason, Terry Nation wrote this stumble into the script, which Hartnell delivered accurately.
This episode was watched by 7.9 million viewers on its original transmission.
