A complaint was received by the Pontiac Division of General Motors that a man’s car seemed to be allergic to vanilla ice cream. Every time he bought vanilla ice cream after dinner, his car wouldn’t start. Any other kind of ice cream and it worked just fine.
The Pontiac President was understandably skeptical about the letter but sent an engineer to check it out anyway. The engineer was surprised to be greeted by a successful, obviously well-educated man in a fine neighbourhood. He had arranged to meet the man just after dinner time, so the two hopped into the car and drove to the ice cream store. It was vanilla ice cream that night and, sure enough, after they came back to the car, it wouldn’t start.
The engineer returned for three more nights. The first night, the man got chocolate. The car started. The second night, he got strawberry. The car started. The third night, he got vanilla. The car failed to start.
The engineer refused to believe that the car was allergic to vanilla ice cream and started taking detailed notes for multiple nights. In a short time, he had a clue: the man took less time to buy vanilla than any other flavour because of the layout of the store.
The car was experiencing a vapour lock. If the guy bought any other flavour but vanilla, the car had time to cool down; if he bought vanilla, the car was too hot and wouldn’t start!