The famous Time Lord travels throughout time and space in the TARDIS. There seems to be very little that this marvellous creation can't achieve, no matter the stakes or the situation.
However, The Doctor has, on occasion, forgotten that at his fingertips lies the technology to shape the universe in ways that only Thanos has dreamed of.
Too often are there stories that have a very noticeable undo button sitting on the wall, yet it is both never touched nor reached for! Sometimes, an explanation is given in the episode. Perhaps the events take place around a fixed point in time and space?
Altering every little moment of history could lead to huge ripples down the line, after all. Yet, as The Doctor arrogantly states himself, he can move through time and save little people here and there.
When he goes too far with this, the Time Lord Victorious is born. Yet, what about the times when undoing the past would not only change things for the better, but save a bunch of needlessly lost lives? Come on Doctor!
10. River Song's Sacrifice
Silence In The Library and Forrest Of The Dead are some of the finest episodes in David Tennant's run as The Doctor, so changing them in any way is heartbreaking. However, once the Vashta Nerada are mollified enough for the survivors to escape, why didn't the Doctor simply nip back in time to warn everyone off? Or explain why the planet was off-limits?
This could have been easily achieved by just dropping some information into the right ears. In fact, over the following years and incarnations, he had plenty of opportunities to warn River herself, as he continued to meet her outside the sequence of time. Why then allow her to die in the manner that he did? And why abandon her, knowing that part of her consciousness lived on in the data core of the planet?
This is never referred to as a fixed point in time and space, so frankly leaving her behind seems to be a cruel move on the part of The Doctor. Perhaps Ten gets the most sympathy here, as he was unaware of the importance of River Song to galaxy. Eleven, Twelve and Thirteen however, that's a different story!