I once cleared a Rem 870 after a friend handed it to me, and a shell popped out of the receiver. It apparently had hung up in the magazine. The look on my friend's face was a study.
Ian McCollom, of Forgotten Weapons confessed to one accidental discharge. A museum allowed him to handle a 19th Century, Colt "potato digger" machine gun, chambered in 6mm Lee Navy. It was nonfunctioning, and Ian, being very knowledgeable about firearms, went through the procedure to clear the action, and got it to clear. He then pulled the trigger, to verify that he had restored the weapon to functioning, and its operational status was verified with a bang, as a 100 year old 6mm Lee Navy cartridge, which had been inside the machine gun for many decades, as it went from museum to museum, went off.
If a 19th Century machine gun, chambered in a cartridge not manufactured for nearly a century, displayed in a museum can be loaded, any weapon can be loaded.