I made a wood burner out of an old propane bottle too. After I cut the top out, for the flue, I left it on it's side for three days, to let any excess gas out. When I stood it up to cut the front out, for the hatch, I thought it'd be best to check all the gas was gone, before attacking it with grinder.
So I stood back and lobbed a lit match in to the top. A 2' flame roared out of it for about 10 seconds. I think that was one of the times in my life that I nearly wet myself.