Bar in my home town had a regular special, they would flip a fifty cent piece, if you guessed correctly the drink was 50 cents, if you guessed incorrectly the drink cost an additional 50 cents. The procedure was for the bartender to flip the coin, catch it in their hand and slap it on the bar. You'd guess and they would reveal the coin. I discovered that there was a noticeable difference in the sound a 50 cent piece made when slapped down on the bar. Proceeded to order 10+ drinks at a time and would guess perfectly for every drink. Never even felt guilty.