When you're eating (usually carelessly) and you spill food on your shirt. Best example is when you're eating chocolate ice cream and it drips on your white shirt (also when you spill a drink on your shirt/pants). Always is an embarrassing moment and you usually don't notice it until someone calls it out.
Cool guy: Dude, check out this really sick Abercrombie & Fitch white polo I just bought.
Friend: Nice! Hey you want some chocolate ice cream on a cone?
Cool guy: Shit yea I do!
(Spills as he eats then goes and chats it up with a hot chick)
Friend: Dude... fail... That's a gnarly Fat Kid Stain on your shirt!
Cool Guy: Dammit you delicious ice cream!