The AA says someone puts diesel in a petrol car or petrol in a diesel car every three and a half minutes on average.
What determines how bad the ramifications are depends on what you do immediately after making the blunder.
If you notice you've topped up with the incorrect fuel before driving away, DO NOT start the car.
Read more: http://www.thisismoney.co.uk/money/cars/article-3572795/Police-forces-spent-400k-five-years-wrong-fuel-cars.html#ixzz48CaXJDsT