As many a driver knows to their cost, putting the wrong thing in your petrol tank at the local garage is an embarrassing business.
But when it's military aircraft you're dealing with instead of a Ford Focus - such a mistake can be very expensive indeed.
That's what red-faced air chiefs have discovered after de-icer was wrongly added to £1.5million of aviation fuel – rendering it unusable.
The contaminated fuel was stored in a giant tanker at the Mount Pleasant RAF base on the Falklands.
Now a multi-million pound clear-up operation has been launched, as the military investigates how the accident happened.
Critics have said there was 'no excuse' for such a basic mistake - the latest in a string of expensive defence blunders.
Last month it emerged the MoD paid £2.4million for rescue and repair after the nuclear submarine HMS Astute ran aground on a training exercise.
Another £1.7million was paid out when a warhead crashed during testing in California.
It paid £1.7million to a company in an out-of-court settlement after it cancelled a furniture contract, and it emerged the same amount was spent on helmets and body armour for Ugandan soldiers.
It comes at a time when the department is having to slash its £34billion budget by 8 per cent, as an estimated 17,000 jobs are expected to go.
Matthew Elliot chief executive of the TaxPayers’ Alliance said: 'Taxpayers can’ t afford the cost of any wasted fuel let alone the millions that his MOD’s blunder has wasted.
'There’s already a huge black hole in the defence budget, it doesn’t need basic, avoidable mistakes like this adding to it.
'There’s no excuse for wasting such huge quantities of expensive aviation fuel, precautions should have been taken to ensure this could never of happened.'
Following the mistake, understood to have taken place within the last few months, a number of aircraft have had to undergo expensive repairs.
They include two military supply helicopters which have had full engine changes. Officials said this was a 'precautionary' measure and there was no danger of the aircraft failing mid-flight.
They also insisted the Typhoon fighter aircraft, which are stationed on the island in case of an Argentine assault, were not affected.
The fuel will not be thrown away, and will undergo an expensive cleaning process.
An MOD spokesman said: 'We can confirm that part of the aviation fuel stock held on the Falkland Islands has been contaminated.
'Aircraft affected by the contaminated fuel have been identified and any necessary repairs have been undertaken.
'Falkland Islands air defence has been unaffected by this incident. An investigation into the cause of the contamination is underway and it would be inappropriate to comment further until this process has concluded.'