What happens when I fill diesel in a petrol car?

It will either not start or it will reduce the octane of the fuel. Reduced octane can cause knocking (detonation), which is harmful to the engine. If it is knocking or sounds at all different don't drive it, or let off on the throttle and let it rev high enough so that it stops knocking. In some engines with carburetors it will ignite itself without the spark plug and you actually can't shut off the engine normally!

If you make this mistake don't drive the car, don't even start it because it's much easier and cheaper to deal with it if the diesel is still just in the tank.

Diesel fuel is denser than gasoline (petrol) so it will sink to the bottom of the fuel tank, as soon as you start the engine it will start to get drawn into the fuel lines, and when it hits the cylinder the engine will quit because it cannot burn diesel. It's not going to explode or destroy your engine, it just won't run. Of course if that happens in the middle of a motorway you could have an accident. If diesel does get to the engine you'll need to drain the fuel system and clean, possibly replace the injectors. If the engine hasn't been run then draining the fuel tank and filling it with the right fuel should be good enough, if a tiny bit of diesel remains it will probably be fine.

Gasoline (petrol) motors cannot combust diesel. You will damage the motor and the vehicle will not run.

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