A propane cylinder could leak, and it's best to assume it does leak. Propane is denser than air and can settle in basins or run along the ground to a source of ignition, then flash-back. It could also drain into a sewer and cause an underground explosion hazard. It can fill up a basement, ignite from a furnace or other appliance, and demolish a house. Facts about Regulators 10.
Every regulator has a diaphragm, a poppet valve and several fittings.
Any of these could leak. In particular, the poppet valve, the diaphragm and the pressure gauge contain mechanical parts. Any mechanical part is subject to failure with use, sometimes suddenly. A separate shut-off valve should be located immediately upstream of a regulator. Regulators are typically designed to handle only gases.