Fuel check questions

[Pinballer]

Being new to all this I see from time to time that somebody doesn't pass fuel check at an event even though they may have bought the fuel right there at the track from the chosen supplier. What conditions and variables are there that would cause the gas to come up testing bad, short of just plain putting an illegal additive into the gas?
What is it that is measured in the fuel check and how do they go about it?