If you find anyone stupid enough to try that then id like to know the answer myself. It would probably blow the whole petrol station killing everybody in a 50ft radius
petrol only really explodes when well mixed with air. When it comes out of a pump it isnt mixed, so if you lit it you would probably get a flamethrower like effect. There would be enough spilled fume to seriously burn the holder, but the true explosion would only follow when enough petrol vapour had accumulated unburnt and mixed with air. It does remain a silly thing to do though.