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A crossover is also a euphemism used by gas jockeys. Like many euphemisms, it's a nice friendly word, without any threatening connotations whatsoever. Except if you work in the oil business, it's quite frankly one of the most terrifying experiences possible.

What it is, is one of two things - either a dozy punter sticks the wrong type of fuel in his or her car, so putting diesel in a petrol car, or diesel in a car with a petrol engine; or it can be the similar but far more serious occurrence of an inattentive tanker driver sending a few thousand gallons of diesel fuel down the tank reserved for unleaded petrol, or vice versa.

To be perfectly honest, the first sort of crossover is nothing to write home about, but if the second sort of crossover occurs, it's a major disaster. The average underground tank at a filling station has an ullage about 7,000 gallons, and the average tanker delivers about 1,500 gallons of each type of fuel total. Now when you have, say, 7,000 gallons of petrol, a few drips of diesel in 7,000 gallons is pretty much a drop in the ocean - literally. But adding such a huge proportion of one fuel type to another is entirely different. The resulting mixture is neither diesel fuel nor petrol; it is pretty much a generic hydrocarbon sludge which is too inert for a petrol engine to run with, and too volatile for a diesel engine to work with. The repercussions of such an incident are understandably immense; anyone filling up their car while the delivery which crosses over is underway will end up pumping more of this hydrocarbon sludge into their tanks, and a large-scale crossover like that can end up initiating several small crossovers.

It's also an extremely expensive mistake as well. If you put the wrong fuel in your car, you'll end up with a bill of about £3,000 or so (depending on the severity of the error) to strip, clean, and rebuild the engine, hose out the fuel lines and scrub clean the fuel pump, replace the fuel injectors, and whatnot. But if the tanker dumps, say, its entire load of diesel down the petrol tanks, it'll probably cost several million quid to put right. For all the underground tanks must be drained and their contents disposed of safely, the tanks and piping and fuel pumps and ancillary plumbing must all be stripped down and cleaned out, the rubber hoses replaced. And that's just the beginning. There's also the loss of sales from the station due to its inevitable closure, which, if it's one of the new breed of petrol stations in Europe with a supermarket embedded into it, can be rather large; not to mention possible litigation from customers whose cars were wrecked by secondary crossovers from the main one. Furthermore, there are tales that some oil companies impose large and, IMO, rather unjustifiable fines on the petrol station which suffered the crossover - even if it was the tanker driver's fault that the crossover happened.

On a personal level it can be just as expensive. The tanker driver who cocked up will inevitably end up staring down the wrong side of a P45, and/or be forced to make a contribution towards the costs of cleaning up afterwards, depending on his or her contract of employment. Similar sanctions may end up being imposed upon the person from the station who oversaw the delivery which crossed over (if that is the procedure at that site.) There will then be a massive blamestorm, and in extreme cases, the site may close.

In general, it is best to avoid crossovers happening, to say the least.