I liked No. 6:

  • If you make a big fuss about sending your soup back because it’s not hot enough, we like to take your spoon and run it under really hot water, so when you put the hot spoon in your mouth, you’re going to get the impression — often the very painful impression — that your soup is indeed hot.  

Link: http://shine.yahoo.com/channel/food/20-secrets-your-waiter-will-never-tell-you-550722/