The doctrine of transferred malice operates to allow the mens rea against X to be transferred and joined with the actus reus that causes the prohibited harm to Y. The 'malice'’ (or mens rea) is transferred from the intended harm to the actual harm. Therefore in the above example, D's intended harm against X can be transferred to the unintended victim, Y, and D will still be guilty of the crime, even if he didn't even know that Y existed.
In R v Latimer (1886) LR 17 QBD 359, L aimed a blow at C with a belt, striking C slightly. The belt then recoiled, hitting V in the face and wounding her severely. L's appeal against his conviction under the Offences Against the Person Act 1861, s 20 was dismissed. The court held that his intention to injure C could be transferred to V.