Natural gas is reactive to the galvanized used in coating the pipe which therefore causing "flaking" inside the pipe. In my area you cannot use galvanized for this reason, even though the gas utility states that "their" gas doesn't do this. There are tons of gas lines and fittings that have galvanized mixed with black iron. Not too often have I heard of clogging directly to this stuation. Not a myth, just a code that covers the "what if".