Overfilled Propane tank is what the firemen said...

The presence of an OPD is not the means by which a propane container is filled. It is only a backup safety device. Since most smaller propane containers do not have a liquid level indicator as the larger tanks do, the bottles must be filled by weight. It is the dispenser's responsibility to calculate how much it will take to fill the tank by weight. Some larger tanks such as those used on fork lifts have a bleeder port and are filled until the liquid exits the fill port.

ryoung