In Doctor Who S9E2, The Witch's Familiar,
the Doctor saves the young Davros by pointing a Dalek gun at the hand mines and exterminating them, and then he teaches Davros about "mercy" to ensure the Daleks will know the word and Clara can prove her identity.
However, during that scene, the Doctor also
shouts "Exterminate!" to preface his hand mine killing spree.
I get that non-canonically, the point was to make an epic cliffhanger (and it succeeded!), but if the Doctor knew that the aforementioned character and his creations would remember everything that was said on that day, then why did the Doctor teach him the word that would become his creations' catchphrase?