Indian Penal Code (IPC)
Section 80. Accident in doing a lawful act
Nothing is an offence which is done by accident or misfortune, and without any criminal intention or knowledge in the doing of a lawful act in a lawful manner by lawful means and with proper care and caution.
A is at work with a hatchet; the head flies off and kills a man who is standing by. Here, if there was no want of proper caution on the part of A, his act is excusable and not an offence.