By Greg Dawson
It's the simplest of mistakes but it could end up costing you a small fortune.
With just a simple squeeze of their fingers thousands of diesel drivers across the country have done hundreds of pounds worth of damage to their vehicles by filling it with the wrong fuel.
Now it's come out that one of Britain's top police forces have forked out more than £40,000 in the past five years repairing the damage done when officers fill up diesel engines with petrol.
To try to solve the problem they've invested in new gadgets they hope will prevent more of the same happening in the future.
Little black boxes have been stuck on the inside of the fuel cap.
When the cap is opened the speaker delivers a short burst of beeps before a message warning drivers they need to fill the engine with diesel only.
New funnels have also been fitted to the fuel necks inside the petrol caps.
The specially designed funnels prevent a petrol nozzle from slotting inside the vehicle's tank which means it's impossible for the wrong fuel to be squirted into the car.
Essex Police say the new gadgets have cost them less than £4,000 altogether and will save them money in the long-term.
These newer diesels are so efficient and quiet that it's easy to forget they don't use petrol
John Gorton, Head of Transport at Essex Police
Head of Transport John Gorton is defending the mistakes made by officers.
He said: "To be fair to them it's a mistake everyone is making at the moment and our officers have got an awful lot on their mind trying to serve the community.
"Officers often drive to work in petrol cars.
"These newer diesels are so efficient and quiet that it's easy to forget they don't use petrol.
"At the end of the day they are human and they do make mistakes."
In 2007 the RAC reported they were called to an average of 130 incidents every day where someone had re-fuelled their diesel car with petrol.
The extent of the damage it can cause depends on how much petrol enters a vehicle's system.
If the fuel tank is drained immediately the problem isn't as severe but if the engine is started, repair costs can range up to £5,000.