Absent-minded policemen have reportedly cost their North Wales force £3,000 in the last year for filling up their diesel cars with petrol.
Last year the force introduced bright yellow filler cap covers marked with the word "diesel". But that did not stop officers making the same mistake 26 times.
A spokesman has told the Welsh Daily Post: "To insert the fuel nozzle, the flap must be physically moved, which should bring the fuel type to the drivers' attention. This has proven to be an effective mis-fuelling device."
Some forces are now installing state-of-the-art light-activated devices that play a recorded message to remind their errant officers that they are driving a diesel vehicle.
The RAC said that in the last decade it has recorded a 385% increase in the number of call-outs to drivers filling up with the wrong fuel.
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