Doctor Who’s first festive special in 2005 introduced David Tennant’s portrayal of the titular hero, who freed humanity from the bloodthirsty leader of the Sycorax. However, he also took it upon himself to dethrone another leader at the story’s conclusion -- the Prime Minister of the United Kingdom, Harriet Jones -- rewriting the future and leading to numerous catastrophes.
"The Christmas Invasion" ended with Penelope Wilton’s Harriet ordering the destruction of the retreating Sycorax ship, killing all aboard to prevent further attacks. Furious at this mass slaughter, the newly regenerated Tenth Doctor whispered, "Don’t you think she looks tired?" to her aide, triggering a media storm about her health that eventually ousted her from 10 Downing Street.
Yet, this is not how her career as PM was supposed to end. After their first meeting in "World War Three," the Doctor, then played by Christopher Eccleston, recognized her as the future "architect of Britain's Golden Age," achieving three successful terms in office. Although the Ninth Doctor would have been appalled by Harriet's actions in "The Christmas Invasion," there is little chance of him altering the course of history to prove his point. By contrast, the Tenth Doctor, who tells Harriet that he is "a completely new man," ended her career prematurely with no regard for how it may alter Britain's destiny.
Instead of the so-called "Golden Age," Harriet's sudden downfall paved the way for the Master, played here by John Simm, to mesmerize the UK population into electing him as Prime Minister under the alias Harold Saxon. From there, the Doctor's nemesis slaughtered a tenth of the human race and enslaved the rest; while the majority of his actions were undone, his assassination of American President Arthur Winters remained intact, causing political instability on both sides of the Atlantic.
By the Tenth Doctor's final story, "The End of Time," Britain and the rest of the world had entered into a global recession, not unlike the one occurring in real life at the time. In response, the new US President, Barack Obama, staged a press conference to announce a financial plan that could end this economic crisis, but this was also foiled by the Master. After being temporarily turned into a copy of the Master, along with most of humanity, President Obama erased the plan from his own mind, dooming the world to further economic turmoil that may have been prevented had Harriet still been the UK Prime Minister.
As for Harriet herself, her time in office was not the only thing to end abruptly, as she was exterminated by the Daleks while helping the Doctor's allies contact him in "The Stolen Earth." Before her death, she stood by her choice to destroy the Sycorax; the Doctor's absence during the ongoing Dalek invasion proved that the world needed to find other ways of defending itself. Though few incarnations of the Doctor would have condoned what Harriet did, the Tenth Doctor might not have faced many of the catastrophes during his tenure had he at least endeavored to see her point of view.
Of course, this was only the first instance of the Tenth Doctor's arrogance getting the better of him. Most infamously, "The Waters of Mars" concluded with him declaring himself the "Time Lord Victorious" after changing a fixed point in time, rescuing the otherwise doomed survivors of a Martian colony in 2059. Tragically, the colony's leader, Lindsay Duncan's Adelaide Brooke, killed herself in order to restore the timeline, dying like Harriet for the greater good thanks to the Doctor's ego.
However, deposing Harriet Jones arguably did far greater damage because nothing prevented it from radically altering established events. Within hours of his regeneration, the Tenth Doctor set in motion a grim chain of events: the domination of Britain and the world by the Master (twice), the recession of the early 2000s and the death and ruin of one of his most valuable allies. With all of this in mind, it is very likely that the Doctor went on to regret saying those six fatal words, "Don't you think she looks tired?"