"slipping up" means that something was on your mind at that moment. it could’ve been brief but you were thinking about it. so at that very moment harry was thinking about a he. and it’s pretty obvious who that he was. and generally after a slip up, you correct yourself or look embarrassed cause you definitely just exposed what you were thinking. i saw NONE of that after he confidently said ‘he already owned it’. you know what he did after? grabbed crotch. it was intentional.