10. Barbara - The Aztecs
BBCNot the biggest mistake on the list by far but an extremely important one in terms of the mythology of the show; the consequences of trying to change history without understanding the ramifications. It's an idea that has reoccurred many times throughout Doctor Who's 50 year history. Even the Doctor fell foul of it, trying to save Adelaide Brooke and the crew of Bowie Base One in The Waters Of Mars, with horrific consequences. Original companion Barbara, mistaken for the goddess Yetaxa, tries to stop the ritual sacrifices of the Aztec people and save the population from future destruction by the Spanish. It might not be as dramatic as the 'Time Lord Victorious' moment, but it is
huge. It showed the audience that the Doctor and his companions cannot use the knowledge of the future to alter time. Just like his warning to Donna in The Fires Of Pompeii, the Doctor understands that allowing this doomed populace to live will change the course of the world forever. What would humanity be like if the Aztecs lived on into the Twentieth Century?
Oh, dont you see? If I could start the destruction of everything thats evil here, then everything thats good will survive when Cortes lands.
It's a mistake Barbara very nearly makes. She stops the sacrifices, putting her in the direct path of villainous high priest Tlotoxl. In doing so she risks the lives of the Doctor, Susan, Ian and herself and threatens the development of humanity as a whole.