text: Step one is ALWAYS (whether it's disassembly or when handing the gun to another person) to drop the mag and then rack the slide to clear the chamber if it's loaded (or to verify that it is indeed unloaded.) You have to drop the mag first otherwise you chamber a round. My guess is that this person carried without a round chambered and didn't drop the mag first and when he went to safety check his weapon he instead chambered a round.