My story: Years ago, while transporting 5 or 6 recently filled propane cylinders (non-OPD) in my van, one that had been over-filled tripped the over-pressure valve. In less than a second I was enveloped in a combination of liquid/gaseous propane. I couldn't see my hands on the wheel. Luckily, it was late Summer and my window was rolled down. Luckily, I was on a straight stretch of rural road with little traffic. Luckily, when I turned the key off, there was no spark. Luckily, I can share this story.
Later, when camped for hunting, we had another cylinder discharge in the same manner. Leads me to believe that all the cylinders were over-filled by the pump jockey. I have never transported any fuel inside a vehicle since that day. If I didn't have a truck to transport cylinders, I'd enlist help or have them delivered.
Note: Today's cylinders can't discharge in the same manner, AFAIK. That doesn't mean that a faulty cylinder can't discharge propane and the gas->air combination reach an ignitable ratio. Be careful out there.