Ok, here goes:
Assuming that you were low on fuel and not empty, lets say that you have incorrectly filled with just over 1/3rd petrol in your diesel car.
Now in an older (pre 2000 approx) Diesel, this might have caused little of no harm, in fact a lot of early diesel users would put ' a little ' petrol in with the diesel fuel to help clean injectors and valves. Petrol, of course, is a great cleaner/degreaser and although you have diluted it with diesel it will mix and lessen the diesel oils' lubricational properties which are VITAL for the diesel pump amongst other things.
So, it kind of depends what you drive. I have dealt with hundreds, yes hundreds of contaminated fuel drains and there are at least 3 manufacturers who will do their best to sell you a new engine following petrol into diesel. They will state that the lack of lubrication will cause bearing failure there-by allowing minute filings into the fuel system,damaging injectors and the cylinder linings... True ..? in some cases,yes - but most filters are good enough to filter these out...? not always.