Relax, said Matt Soniak of mental_floss, the engineers have actually thought of this possibility and designed the gas pump accordingly:

Rest assured that you won’t be causing any fireballs or explosions. The hose that attaches the nozzle to the gas pump is designed to break into two pieces when a certain amount of force is applied to it. Next time you’re at the gas station, check the hose for a metal coupling. That’s the break-away point. Once the hose is broken and you’re off on your merry way, check valves in the hose keep fuel from leaking out and creating a hazard.

The financial hazards, however, may still sting you at the wallet: Link