This is when you fail to obviously mark consumable items (e.g. shampoo, cereal, etc.) with your name, thus leading your roommate/family members to believe that the item must be for them.
You: Dude, why did you eat all my Pops?!?! I just frickin bought those!
Stupid little brother: Well I don't see your name on them, so you were sharing by default.
You: How about i share my fist in your face?