After filling a propane cylinder at a service station, the attendant placed the filling nozzle on top of the propane piping to hand the cylinder to the customer. The nozzle dropped. The impact with the ground reopened the control valve and resulted in an uncontrolled release of liquid propane. The dispenser lacked the required excess flow valve to control or slow the propane flow. The attendant was able to shut off the propane flow to the dispenser, and the propane eventually dissipated before coming into contact with any source of ignition.