Petrol in diesel engines isn't a big issue but the other way around is disastrous. Modern petrol engines are filled with gas-sensors and temperature probes that are very sensitive to their environment. I know someone who accidentally ran a petrol engine on diesel about 12 months ago. The engine hasn't run right since and that's after replacing lambda sensors, co2 sensors, ECU etc. They just don't seem to be able to get to the bottom of what the problem is.