Emma bought a ticket for a day trip to London, saying that she wanted to travel on the next train, leaving in 15 minutes. When the inspector checked her ticket on the train, she asked Emma to pay a further £6, as she had been undercharged by the booking clerk.
Although Emma claimed that it was the train operating company’s mistake in selling her the wrong ticket, by law she had to pay the difference. When a contract is made, one side cannot gain by the other side’s genuine mistake.