Almost similar attempt was made when I went to fill my car on 12.7.2012 at 7 PM
I had asked to fill 20 litres.
The mag was there. I didn,t get distracted.
He then filled 5 litres and asked if want to pay cash or card. When I reminded him that I wanted 20 Litre, he promptly filles another 10 litres. The meter showed 15 litres. He said 5 litres first time and 15 now add up to 20 litres. When I sternly looked at him, he said oh sorry and filled another 5 litres.