A fairly despicable habit originating in student bars in the UK. After the bar is shut, but while people are still in the venue, the more desperate alcoholic will go round collecting any half-full abandoned drinks he can find, and neck them without anybody noticing. The more drunk you get, the more likely it will be that you drink out of a glass that someone's used as an ashtray. Another possibly hazard is rohypnol
Tom: Hey buddy where d'you get those drinks from -the bar shut half an hour ago?
Dick: Oh I just went minesweeping round those tables at the back.