Missing petrol

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During the gasoline transporting from one plant to another,
the fuel tanker ever wasted 10 liters of gasoline.
The process of filling the tank at one plant and discharge
of gasoline from its bottom at another is monitored very carefully.

All holes of fuel tanker were thoroughly filled, and it was accompanied
by a reliable guard during their journey.
But  still, the discharge of fuel  always lacked exactly 10 liters

Where did the petrol disappeare?