If you stand say five feet away from a mirror and focus on your reflection with a  manual focus the lens will focus not to a distance of five feet, but rather 10 feet. Even though the mirror is a flat two-dimensional surface, optically the virtual space behind the mirror must be accounted for. I remember being amazed when I learned this fact many years ago. Mirrors have always seemed otherworldly to me.

This image is a reflection in a polished metal sculpture in downtown Toronto, and is a heavy crop from a much large frame. For me it still has the magic of a mirror though.