This went on for about a week until people realized there was a safety problem. When gasoline is mixed with diesel, the vapor pressure changes and the flash point, which is normally about 150 F, drops to 60 F. That creates a combustible mixture that can cause an explosion. Fortunately, diesel manufacturers learned this and warned truckers before there was an accident. There can be a flammable mixture in a tank, but you need a source of ignition before there's a problem. Ignition sources can be just about anything.
Sparks can develop from an electrostatic charge just by pumping fuel into the tank.
During a rainstorm, lightning can create an electric discharge by creating an electrical field across the tank. It doesn't take much energy. The Air Force is interested in finding a good handheld detector to measure the vapors of JP-8 fuel in fuel tanks and fuel spills. Detectors can measure simple gases such as methane, pentane, and propane, but it was questionable whether they could measure JP-8 vapors. We built a facility to create those vapors and test the detector instruments.