I've posted this here before and someone should remind me to post it more often…
There is forged original artwork out there. Fake. Bogus. Fraud. Not actually by the person they say it's by.
Phony art is usually not in the form of something that was published. It's an alleged rough sketch by some famous artist or a drawing that was supposedly done for a fan. Lately, I've seen a number of these. Someone traces an obscure Jack Kirby drawing and claims the tracing is by Kirby. Or they copy a panel out of a Peanuts strip and say it's an original sketch by Charles Schulz. (Not long ago, eBay had one up that was signed by "Schultz.") Occasionally, one sells for an awful lot of money.
Kirby and Schulz seem to be the most oft-faked but I've seen others…and an awful lot of fake animation cels. I've stopped authenticating artwork but there are reliable art dealers who can tell you if that Jack Kirby sketch you purchased of Obama's inauguration is legit. Be on the lookout.