It was a sunny day at the petrol station. The birds sang, leaves rustled in the breeze, and an angry man swore and pounded his fists on the bonnet of his BMW. Some say he had just dropped his keys in a drain, others argue his wallet had vanished, but he had actually just put petrol in his diesel powered car.
This is actually quite common. I know literally 'some' people who've done it. The first thing to remember is the sun will rise in the morning – so don't PANIC! The second thing is to avoid starting the car. Just slide it into neutral, release the handbrake, and persuade a beefy bloke to push it out the way.
The next stage is to ask a mechanic to drain the fuel tank. As long as the car isn't started that should be sufficient. If it was started you may need new spark plugs and filters. Either way, unless you're unlucky it shouldn't be the end of the world. At least you'll have an interesting story to tell your friends.