30-year-old David Walter strolled into his local Walmart with a semi-automatic handgun in a holster strapped to his hip. (He was also wearing an Arizona Department of Corrections shirt. According to the Department he had never worked for them, but had once attended two weeks of a longer training session.)
Witnesses said as Walter walked through the store he repeatedly messed with his gun, taking it in and out of the holster, taking the magazine in and out of the handgun. As he stood in the checkout lane he again pulled the gun from his holster. The magazine fell to the floor and when he inserted it back into the gun the gun discharged, firing a round into the ceiling.
It should be noted that Walter broke no law by walking into Walmart with a gun openly visible on his hip. In fact, some form of open carry is legal in over 40 states.