When you try to make your presence known to someone by waving or nodding, the the recipient either ignores or does not realise. You have just been pied. This is the perfect time for a friend to suggest that you wipe your face, because it's covered in pie.
Two people passing one another,
"hey man how's it..." as the other person didn't realise and looked the other way; he has accidentally pied his friend/colleague.