Today my wife arrived home in the old Polo which you still have to lock with a key. She had only done a couple of miles since she last unlocked and only past 2 cars on the way home. She came in doors and said I cannot get the key to go into the lock. I inspected, and there was a stone inside the slot that the key enters. It took a while to extract the stone, it was just slightly thinner than the slot and about the same height as the slot, and about 5 mm deep. It must have been thrown up by a passing car.