Yes the wife has her first diesel car at the moment, and she's terrified of putting petrol in it, but i've told her that as long as she has not put more than say 10 litres in before remembering, filling up with another 40 of diesel to a full tank shouldnt really hurt it, the theory has yet to be tested, though. I know it doesnt make any difference with other diesels I work with all the time, but they are more basic injection systems, I havent tried it with a 2 year old car yet
. Infact I've seen much higher amounts of petrol than that, and they still run fine, albeit a bit hot. Its when they get to about 55 -65% petrol that they start to conk out, and even then its not some explosive spectacular, they just wont run.