Does one mistake cause one to be "accident prone"? Especially when that mistake occurred while attempting to comply with a rediculous law?
My opinion, not that it's worth anything....
Write the guy a ticket for a $100 fine for negligent discharge of a firearm in a public place and move on. Then, if there is a second occurrence, suspend his permit pending completion of gun safety training. On third occurrence revoke his permit permanently and, in a state such as Washington - force him to carry his gun openly without a permit!