When your friend passes out from drinking, and someone draws words and pictures on their face/body. They awake the next morning, with visible reminders never to drink that much again.
When others treat you with disdain or contempt for (1) not wearing a mask in public or (2) wearing a medical grade mask that health care providers need.
I was mask shamed while lining up for groceries without a facial covering. The piercing stares and angry mumbles from those wearing masks left me feeling uncomfortable.