Have you ever heard of "misfuelling"? If you live in England, chances are you have. It's the practice of putting the wrong fuel in your car - either diesel in an unleaded vehicle, or unleaded in a diesel vehicle. Last year there were over 150,000 incidents of misfuelling in the UK, and it's become so common that you can actually buy insurance against this practice. Insurance runs about £40 a year which is cheaper than the £200 or so that it will cost to get a roadside assistance service to pump the tank out. Which in turn is cheaper than the several thousand it will cost you if you start the engine and drive off.
So how does this happen? Honestly I've no idea. For a start, the nozzle and fuel filler diameter is larger for diesel pumps and diesel fuel tanks. This means that if you use the unleaded pump, the nozzle is going to be rattling around loose, without a tight fit, and won't hold itself in the filler. That ought to be a clue. The smell ought to be a clue too - diesel smells radically different to petrol. The colour of the pump and pump handle should be a key reminder - unleaded pumps use green handles, diesel use black (unless you're in America in which case it's the opposite). And the fact that you're at a completely different pump ought to be a giveaway too.
But for one person every three minutes, none of these things trigger them to realise they're about to incur a very expensive mistake. Ok realistically I can see how putting unleaded in a diesel tank can happen - the nozzle fits, so go for it. But believe it or not, this mistake does happen the other way around. Despite the diesel nozzle not fitting an unleaded filler neck (because the nozzle is too big), people actually manage to fill their unleaded vehicles with diesel by "aiming" the nozzle, or jamming it against the filler neck and trying to keep the nozzle aligned with the hole. Frankly if you're dumb enough to do this, you deserve the expensive bill.
So what happens when you "misfuel"? Loosely speaking, diesel is a lubricant with more oil content than petrol. Petrol is a solvent. So if you put petrol in a diesel engine, the solvent strips all the lubricant away from the engine and the metal parts weld together. If you put diesel in a petrol engine, you'll get a thick blanket of white smoke out of the back of the car and all the fuel lines and injectors will gum up with oil.
Whichever one happens, the minimum penalty for doing this will be a complete new fuel system - tank, lines, pumps, filters, injectors etc. Look at around £3000 as a starting price for that. The maximum penalty is a new engine, in which case you might as well junk the car.
The cheaper option of course is to pay attention when you're filling your car. Although given how few people pay attention when they're actually driving nowadays, I suppose it's only natural the the cellphone conversation or texting will take precedent over making sure you put the right fuel in your vehicle.