I used to work in a thrift store, and one time we had a customer who wanted to be let into the store 10 minutes early. We told him no, and eventually he came back when we actually opened. He then started ranting to us about how customer service should work, and how we were going to go out of business because we didn't know how to put the customer first. He went on to say that he was smarter than all of us because he went to Harvard.

Bonus: when he went to check out, he was buying a teacup that was marked $5. He asked me if I could lower the price, and when I told him that all prices are final, he answered with this gem, "It's always no with you isn't it?"