When it happened to me, the front desk changed my preassigned room at check in to give me a better room with a view. I suspect the front desk agent forget to correctly/completely change the room assignment in the computer. When the guy who had been preassigned the room I was given, the front desk gave him a keycard to the room I was given. I had secured the deadbolt and was taking a bath when I heard rattling of the door. I loudly asked who was there and he was clearly shocked. He went back down to the front desk, who called me to ask who was in the room. I kept the room and the hotel sent a bottle of ice wine and a place of chocolate-dipped strawberries with a very gracious handwritten note of apology on a silver tray. Due to the nature of the hotel, its location, and the guy's polite voice, I didn't feel seriously threatened.

BTW, at least in the most commonly used keycard technology in the USA, whenever the hotel makes a new keycard for a room---either to assign it mistakenly to someone else, to make an additional key card after check in, or because a key card was lost---all older key cards for the room become deactivated as soon as the new keycard is inserted to attempt to enter the room. This is why the initial occupant of the room must get new key cards from the front desk. The system prevents two parties from ever both having valid key cards for the room.