A while back (perhaps a year or so ago) I read a book called "The Card", about the history of the world's most expensive baseball card (a T206 Honus Wagner card, made between 1909 and 1911 and worth hundreds of thousands of dollars.) The book discussed in detail questions about the card's authenticity (some suspect the card might have been doctored to make it look more pristine) and about the authenticity of baseball cards in general (there's no way to prove provenance in the hobby, and historically, the cards have been low-quality and easy to manufacture...and hence, easy to fake.)
This led me to the book "Card Sharks", which discusses how that exact problem (forgery and fraud of a product with no trail of provenance) led to the creation of the Upper Deck company, which makes cards that are higher-quality and harder to forge...and how some people suspect that Upper Deck themselves has been "forging" their own cards by creating a larger number of cards than advertised, and quietly disposing of the extras on the secondary market. (In other words, you tell people that "only 1,000 of these cards will ever be made!" Then you print 2,000 and sell the extra thousand to collectors on eBay for ten grand each, knowing that the odds of anyone actually being able to piece together a definitive picture of who's got which cards are so low as to be non-existent.)
And then recently, I read "The Billionaire's Vinegar", which is all about the world of wine collecting, and how (again) there's no such thing as a trail of definitive provenance and no ability to definitively prove the age of a bottle of wine (the best they can do is prove it came from before World War II, because nuclear testing has deposited microscopic amounts of radioactive material all over the planet since then. Cheery thought, hmm?) So again, forgery and fraud are rampant, because there is no way to prove what the definite article is.
And this combined with some thoughts of mine about sports memorabilia (where it's well-known that fake signatures circulate on baseballs, bats, jerseys, et cetera, because there is...all together now...no trail of provenance for most of those items. Yes, a lot of modern signed products come with a "Certificate of Authenticity". But, well, Upper Deck proves that those are only as helpful as the people they come from...always assuming they're not forged, too.) And I've come to a conclusion.
There have to be fake comic books out there. It's just a given. You've got a hobby with no means of establishing a definitive trail of provenance (comics are passed from collector to collector, and collectors routinely buy from other collectors with no way of guaranteeing that the comic originally came from the company.) You've got a product that is easily forged (the printing process on older comics was cheap and easy, by design.) And you've got an economic incentive to forge. (Amazing Spider-Man #1 sells for upwards of $40,000...that's a pretty big reward for forgery.) Sure, there are "authentication services"...but go read those three books, and you'll get a pretty good idea of just how asymetrical the war is between forgers and authenticators, and just how likely it is that the forgers are winning at any given time. (For those of you who don't have the time or money to read them, I'll just say that the answer to both questions is "very".)
So in other words, if you're a comic collector, particularly a big-time comic collector...odds are, at least one of your showpiece comics is a fake. And you'll never know which. Fun thought, huh?