Seneca, On Benefits
4.34.2 (tr. John W. Basore):
For the same thing may happen in battle, and my hand, deceived by some mistake, may direct my weapon against a comrade, and spare an enemy as though he were a friend; but this will happen but rarely, and from no fault of my own, for my intention is to smite the enemy, and to defend my countryman.
sic enim in proelio potest accidere, ut telum meum in commilitonem manus dirigat aliquo errore decepta et hosti tamquam meo parcam; sed hoc et raro accidet et non vitio meo, cuius propositum est hostem ferire, civem defendere.