When I first moved over to diesel, I managed to put a few litres of petrol into the tank. After a panicked call to a car club friend of mine, he told me just to fill up the rest of the tank with diesel and let the petrol dilute.
Car drove away fine without any ill effects. I'd guess that a few litres in an 80 litre tank would dilute quiet well. But then I don't know how sensitive modern engines are or what the safe dilution ratio is.
I've never mis-fueled since then, but I'm very careful in checking the colour of the pump before I pick it up, also making sure I've got the cheaper option and not the high-performance, overpriced version.
I also do daft things like catch myself from pouring milk into the kettle or trying to make coffee with cold water...
but as far as I'm aware, misfueling a diesel engine with petrol isn't as damaging as the other way around (which explains why you can't put a diesel nozzle into a petrol car).