Answer: If by mistake one fills petrol in the diesel engine damage will happen for sure. Petrol is volatile, hence, if one tries to start the engine it would lead to detonation as the compression ratio in diesel engines is almost twice as high as in petrol engines.
Also, diesel acts as a lubricating oil hence if petrol is added to it, it will lose its lubricating property and wear the engine parts in a short period of time.