I've done the same at least twice at gun shows.
A friend who used to own a local store (sold out recently) picked up a gun from the counter to show and found a loaded round in the chamber. This guy ALWAYS checked the chambers on any firearm before handing them to a customer with the action open.
Earlier in the day he had shown that gun to someone else. When the handed it back he put it back under the counter without re-checking. He is certain that guy dropped a round in the chamber while looking at the gun. Not sure why. After that day they all got checked before putting them up.
