Doctor Who: 20 Things You Didn't Know About The Day Of The Doctor
14. Why The Eleventh Doctor Forgets The Children
The moment that Ten scolds Eleven for forgetting how many children were on Gallifrey the day he destroyed it was chilling.
In the onscreen version, Eleven justifies this by explaining that he's "moved on", but the novelisation explores his forgetfulness a little deeper.
It explains how the Ninth Doctor spent so long saving life after life to distract himself from counting. One of those lives saved was a therapy robot, which he then let loose in the corridors of the TARDIS.
The novel suggests that, years later, River and Eleven found the robot, and River used its power to erase the Doctor's memory of the children on Gallifrey:
"'You tell it your worst story, and it edits the nasty bits out of your memory', River smiled, that way she did when she's done something she shouldn't have - which in fairness was the only way she ever smiled."