Character mistake: The Doctor gets Jo to do some mathematics to distract her. At one point she says "49 x 2=88", when it actually equals 98, then she says that "adding 10 equals 108". (In other words, Jo (Katy Manning) fluffed her line, got one figure wrong, then went back to script...).
Character mistake: The Doctor (Patrick Troughton) makes one the funniest fluffed lines in the entire history of Doctor Who the final episode. With all sincerity he says "We'll all be dead soon, unless you switch over to the SEXUAL air supply". (The word Troughton was probably looking for was "sectional").