A instigates a child to put poison in Z’s food and supplies the poison. Acting under that instigation,…

Section 51 and its mistaken-food illustration make the abettor liable for a different act where it is a probable consequence of the abetment and is done under the influence of the instigation, so "A is liable for the act done as if A had directly abetted poisoning Y's food". The mistake does not exempt A, so "A is exempt solely because the child selected the wrong food" is wrong; liability for the act done does not create a completed murder without a death, as "A is automatically guilty of Y's completed murder even if nobody dies" suggests; and "A is liable only if A personally places poison in Y's food" is wrong because personal administration of poison by A is unnecessary.