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.
Mirror tests tempt me to bring seven years of bad luck upon myself
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
"We were so desperate for employees we just gave them the mirror test and hired them."