The requirement to visually check fuel quantities before flight was brought home to me in a salutary fashion a couple of years ago. I am the only person that flies my aircraft and I keep meticulous fuel logs and complete visual checks before flight.
The aircraft normally resides in a hangar on a secure airfield overnight however on one trip I made – I left it overnight on a smallish rural field in UK and returned the next day to find that someone had stolen about 400 litres. The 4 tanks were refuelled after arrival ready for an early pre-planned departure time the following day to meet a slot. That flight was to Germany over the North Sea.
So even after confirming the uptake from my fuel log during the standard daily A check and knowing that I had uploaded (and paid for) full tanks the previous day – I could have been caught out unless the visual check was made.
I guess it confirms Eugene Fordsworthe’s premise that “assumption is the mother of all screw ups” – although I believe he later qualified this adage to “I have assumed that there is no good in assuming."