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I hear a lot about the problems that can occur if you put petrol in a diesel car! But most from my friends, who really don't have a first clue about cars! Is that really such a problem or are they just talking rubbish? I drive a petrol car, what about if you put diesel in a petrol?  

First you have two completely different combustion values, gas is much more explosive than diesel and burns at a greater heat, I have played with putting deisel in a gas burner at a fifty percent mix, you get a great deal of smoke and lower performance because of the lack of bang, lowering the octain. this was done oh some thirty years ago so long before the ox sensors and cats. diesel requires a much higher compression to work well. compression being higher would cause the gas to burn the piston in a diesel if it would ignight. I hope I helped you in some way.  

Thanks for the information Fordruner, you did help me. But do you think that if you put some petrol in an almost full diesel tank, (by mistake) will that mixture also be fatal to the engine? like 80 % of diesel and 20 % of petrol?  

hm..I think 80 % can be too much. If we should consider 10 % than maybe the engine can survive, but you should take a car to the mechanic if something like that happens.

The problem is real, because I have also have some friends, that were careless and did put the petrol in diesel, but all of them noticed it before they started the car on the Petrol station. There is an easy way of preventing the misfueling problems, by buying a device that doesn't allow you to put the petrol nozzle in a diesel tank - www.solodiesel.eu.
hope i helped you in some way
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Twenty percent is kind of high, but in days gone by we would blend white gas(unleaded) with the diesel in our trucks, this done in cool weather. that helped to keep the fuel from gelling. we put between one and five gallons per hundred gallons, more for colder days, less for not so cold.
  The best thing to do is to drain save and put a mix of half and half, this way you can use it and not waist it.  The only other thing you can do is to keep a very close eye on your temp. if it is a turbo system and you have a pyrometer, that gauge should be the main unit to keep an eye on, it will tell you faster if your heat is to high.