I almost had something like this happen when I was finishing up my current home, and about to move in. I have a 500 gal. propane tank buried in my side yard. I was the GC and we are in the county, so no permits or inspections, but still, you have industry safety standards.
My tank runs to the house at no more than 10 psi, it is then regulated down to 4.5 psi, before finally being regulated down to 0.5 psi. Propane isn't supposed to enter the house at more than 0.5 psi, all gas appliances, fireplace inserts, etc. orifices are sized and designed to run at 0.5 psi.
We were about 3-4 days from moving in, and we were cleaning the kitchen. I decided to try a burner out on my new cooktop. I reached down and turned the gas and hit the striker at the same time. When it lit, I had a fireball that blew the big pan drawers completely out to the end of the tracks, the underside of the cooktop was on fire, somehow, I was able to get back to the burner control knob and turn it off. I didn't have hair or eyebrows left. But the fireball was brief. It burned up just the sawdust that was still in the drawers, and went out. We had to have the cabinet sealer sanded back down, resealed it and everything was fine.
Next morning I called the propane guy who had installed it, told him what had happened, and you could hear a pin drop. He, and his crew where out here in about 30 minutes, and went through the system. They had forgot to put in the valve that metered the gas flow to 0.5 psi, so I had 4.5 psi going to the cooktop. With seeing what happened with just 4.5 psi hitting air, after being on for a split second, I can say that if a valve had been left on for 5-10 minutes, and somehow got lit, it would have caused that kind of devastation. Even at 0.5 psi, and someone carelessly leaving a valve open for a day and it gets lit, you are going to have that kind of devastation, especially if the house is sealed up with minimal to no leaks.
CHIEF