A rule three violation...

...or more likely simple ignorance by the officer of what he was picking up off the pavement? After making an arrest the officer noticed an object on the ground and picked it up.

When Pence picked up the [zip]gun, it fired, injuring his hand and causing powder burns on his face and hands, DeGeare said. Pence was treated at a local hospital, DeGeare said.

More likely the second with just plain bad luck mixed in.

H/T to The Duck.