When you go to pack a little bit of keef from your grinder on top of your bowl and realize there was not as much as you thought. The keef thief could be a.) some grimey bastard at the party you had then night before, or b.) the result of your high ass spilling your keef without realizing it.
Rob: Dude I coulda sworn my keef collection was bigger than this. Fuckin keef thief strikes again.
Alex: Actually your dumbass spilled half of it while you were admiring your stash and didn't realize it. Moron.