If you try hard enough you can pour any liquid into the tank.
If you have done so and it is just a splash or two I'd suspect you could dilute it enough with petrol not to have a problem. But phone up your road assistance company of choice and confirm.
Thanks for the answers. It's a debate that's going on between myself and a friend. I thought that it was virtually impossible due to the diesel nozzle being bigger and not being able to fit. Is this true?
One way to find out. Take your petrol car to the station, and accidently pick up the wrong nozzle to fill with, but remember not to actually pull the trigger. Realise at the last moment and get the other one instead.
Some cars, but not all, have a tank nozzle that will not allow the incorrect fuel nozzle to fit - but it's not worth taking the risk with those that don't.
Diesel nozzles are the same size as the old four star nozzles, unleaded petrol nozzles are smaller, so you can fit an unleaded nozzle into a diesel car filler, but not the other way round.
Of course if you try hard enough I'm sure you could get the wrong fuel in a petrol car!
Before the Diesel pump whas in line with petrol pumps on the forecourt derv was on its own & more often was used by HGV's the nozzle was larger in circumference & would not go into a petrol tank of a car.