Even with good equipment, I find it unlikely someone could make a fake one that would past the average owner's muster. Several of us are probably quite familiar what these look like, both tarnished and untarnished and would not be fooled, even by a good fake. Some of us have even opened these ourselves before.

Years ago, I gave away about a dozen of the 2011 error 1BTC coins to friends and family and have told them several times that if they go to sell them they should let me know, then reference me and my forum name so that I can personally vouch for the authenticity of their coin. I like to think it might help, but I don't know for sure if it did because so often it's hard to tell if the legit sales on here are actually legit.

The only legit sale I know of was one I recently (this last summer) had a friend do on Ebay because he insisted he sell his coin.