[jumpy]

Older mechanically fuel-injected engines will run quite happily on veg oil (with a small percentage of normal diesel added to the mix) however many of today's more complex diesel engines could suffer damage to things like the fuel injectors and the high pressure injection pump; mainly this is due to the higher manufacturing tolerances in these components these days.

Like the old leaded petrol engines, some parts of diesel engine systems rely on the natural lubrication properties of diesel fuel to prevent parts suffering from excessive wear.
So don't make the mistake of putting unleaded into your modern diesel by mistake then driving it - you will kill the injection pump because the petrol will act as a solvent and strip all of the lube from inside the pump. If the pump itself doesn't completely fail the at the least you run the risk of bits of metal getting stuck in the injector nozzles. Which will knacker them!