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.