When I worked at Ford a guy brought his mondeo diesel in as it cut out. All the time he was asked he denied putting petrol in as per the norm. He even had his receipt which confirmed he used the diesel pump. After stripping down the tech took a sample of the fuel. Found petrol mixed with the diesel! We had to write a report on our findings as he was putting in a claim for the repairs. The petrol station.............Tesco!!!

Never did hear what happened with his claim though.