Hotel bar, night of a wedding. Young guy ordered a shot of Patron. I served it then turned away to help someone else. I barely registered an odd noise from his direction, but was too busy to really notice it. I finish up and turn back around, kid is gone, cash is on the bar next to his used glass. As soon as I look at the glass I realize what the noise I heard had been. When he took the shot, he immediately puked it back up, into the glass and bounced. I used a napkin to grab the glass and tossed it in the trash. Never saw that kid again.

Volunteer bartending while traveling in South America. Loooooots of cocaine. A regular event was to close our hostel bar at 2am, do a "comunity line" where you set up a long enough line for everyone in your group to take it at once on the bar top. Then we would hit the club bars, more coke, hit the beach bars at 5, then walk home at sunrise once it was safe to walk on the beach. That is were I learned the catchphrase "cocaine is the vitamin that helps you drink more!"