I have never heard of someone getting the wrong gas (though it really doesn't surprise me that it happened!), but I went into the LWS a while back to exchange a cylinder of mix (Argon/CO2), and the guy working the dock tried to load a 250CF tank into my truck (I exchanged a 150CF tank). Of course I have a conscience and mentioned the error immediately (I'm sure they would have noticed it on their own sooner or later anyways), but it sure would have been nice to have the bigger cylinder.