When I was in Uni I met one of my housemates at the train station, he asked me if I wouldn't mind taking a small rucksack home from town. It was strangely heavy for it's size.
Got home and left it on the kitchen table. Later that night, and in the middle of a drunken gathering, talk turned to the rucksack, the housemate in question opened it and took out a gun. He said it was a cap gun that his grandfather gave him when he was a child. We all had a laugh messing about with it, got very very drunk and eventually crashed out.
For the next two terms the cap gun found a home on the coffee table, and everytime someone came home, whoever was closest to the table would grab the gun, point it at whoever had just come in and deliver their best/bad movie monologue (think dirty Harry etc). It became a sort of tradition in the house.
One day, near the end of the year, I got back home to find two police officers standing in the hallway, and my housemate looking very sheepish. Apparently he had taken the cap gun into an AmDram society rehersal that he was involved with. One of the other members examined the "prop gun" and told my housemate that it was infact a WW1 revolver, very real, not a toy, and it needed to be handed over to the police.
I'm guessing the same guy actually called the police because two officers showed up at our house the following morning.
My housemate was given a telling off, but not (as far as I know) charged with anything and the gun was taken away.
I can't imagine what would have happened if one of us had been sitting on the couch and pointed the gun at them as they walked into the room.