When a screw sticks out on metal or isn't perfectly flush with the surface, an angel in heaven loses an eyeball. Or something like that. But fret not eyeballs of angels in heaven, people of Earth can make a screw completely flush. It's a lot of filing and sanding and hammering but eventually the screw disappears.
Of course the screw hole is still there but still, I'm counting this as basically invisible. The work starts at around the 13:20 min mark in the video of a Holland and Holland shotgun being made: