My former boss loved to cook and frequently brought in sweet treats that she had made at home for everyone to share. In particular, she loved to make candy.
On Valentines day, she brought in home-made chocolate covered cherries and walked around the office sharing them with everyone. What we didn't know is that she has secretly covered some olives in the bunch. Approximately 10 out of the 50 were chocolate covered olives.
So desk by desk she shared the yummy treat and everyone would compliment her on how great they were (that is, everyone who actually got one that had a cherry inside.) You could always tell by the facial expression when someone got the olive. Not one person spit it out, you could just tell by their face that it didn't taste good. When she told everyone what she had done - the poor souls who had got the olives raised their hands to say - "That's why it tasted so aweful, when everyone else was saying how good they were!" It was hilarious!