In Canada, a major chain of coffee shops (didn't know if I should/could say the name) has their annual roll up the rim to win for prizes. If someone bought you a coffee and you discovered a prize of substantial value under that rim, like a new truck, what would you do? or If you bought the coffee and that person won, what would you do? This applies to any gift with the possibility of winning a prize.
BTW, I haven't encountered this situation, just thought it would be fun to discuss.