It is possible. An example would be if Aggron used Head Smash on Blissey. Let's assume Aggron EVs are 252 Att and 0 Hp, and Blissey EVs are 252 Hp and 0 Def (perfect IVs all around); both have neutral natures. According to Smogon's Damage Calculator, Blissey would get OHKOed, and Head Smash would damage Aggron with half the recoil. Half of Blissey's Hp is 357; Aggron only has 281 Hp, so Aggron would get KOed.
But your probably safe if the attacker isn't using Head Smash and the defender isn't Blissey/Chansey.