|The "dirty-public-restroom selfie"|
It usually goes something like this:
Amanda strolled through the hall with a hop in her step, turning the corner and catching sight of herself in the mirror: her long strawberry blonde hair falling down across her thin shoulders, light freckles sprinkled across her pale face and framing her fiery honey eyes.
This is a moment in the prose borne only to familiarize the reader with how the character looks; a chance to list out his or her physical traits in a quick and compact fashion so as to get on with the great business of storytelling.
And mirrors offer up such temptation, for many of us see them everyday in our lives, when we prepare ourselves for work and school and going out, whenever we go to the bathroom, whenever we are feeling unsure of our appearance. It's easy enough to simply have our characters nonchalantly stroll by a conveniently-placed mirror, take a quick momentary glance, and commentate on how she or he looks (as many of us do) at that moment. Because, what reader would care about our characters if they couldn't know exactly what they looked like?