A test performed to determine if the subject is breathing by holding a mirror in front of their mouth/nose to see if it forms fog

When you eat dirt for the first time and leave your family to build a shack in the woods

An artist's worst nightmare, especially if they draw people or portraits.
This is when you flip over a drawing and hold it up to the light to see to the other side / use a handy mirror to check if your image still looks symmetrical when reversed.

Usually, it looks fine from the front and fails the mirror test.
It's really discouraging.