I expect that they take the pilot's word in case of a rare situation [1] and then make the fill a ton of paparwork to try to solve the main cause and also discourage lies.
[1] In one case someone mixed imperial and metric unix, and instead of $something-kilograms, they put only $something-pounds of fuel.
This incident is known as the Gimli Glider and was actually due to multiple failures before the pound-kilogram issue (and the backdrop of Canada's then-recent metrication) even became relevant: https://en.wikipedia.org/wiki/Gimli_Glider