a friend of mine had a fitting leak on propane heater after changing out tanks and it exploded on him. he is in hospital with burns over most of his body.
must remember to keep spare tanks warm and make sure no ice has formed around fittings of both tanks and units being used. ice chrystals form on necks of tanks when they are cold as you hook up a new tank. these chrystals cause propane to leak out around fittings.
a couple of years ago my son-in-law, his dad and i were out chasing crappy. one of the lanterns ran out of propane. the s-i-l change tanks and he ended up frost biting his hand when the tank wouldn't seal up and the tank leaked onto his hand.