Someone who opens a beer, takes a sip or two and then just ignores it. Since others know that the beer is probably owned by someone, they don't drink it either. After thirty minutes or so the CO2 is gone from the beer and the beer has warmed up to room temperature, making it unenjoyable to drink.
This usually happens when people also smoke weed during the occasion. It should be punished by death.
Guy at a party who finds a near-full but opened bottle of beer that has been standing there for over an hour: "We've got a beer murderer in the building!"