If a creature starts to move, triggering an opportunity attack as it does so, can it then choose to not move as a result of the attack?
As some context, picture this: a goblin is in melee with an Eldritch Knight fighter who has the Booming Blade cantrip and the War Caster feat. The goblin turns to run, at which point the fighter can make an opportunity attack. War Caster allows the fighter to cast a spell at this point, so he chooses Booming Blade. This deals some damage immediately, and sheaths it in booming energy which deals extra damage "if the target willingly moves" before the start of the fighter's next turn. Seeing this energy surrounding them, can the goblin choose to stop moving and avoid the damage, or is it too late for them to make that decision, making the damage inevitable?
In fact, the link to the twitter post answers your exact question.
– Airatome Jan 22 '16 at 22:09