Just read an interesting article towards the back of the March Numismatist about an 1823 large cent that got sent in to NCS for conservation. I wish I could post a picture of it but I'll do my best to describe it for anyone who doesn't get the magazine.
Nothing stands out on it as being a fake. It has the normal, chocolate brown color to it. But on the obverse from about 6 to 9 o'clock they discolored it with a stain and added some black staining to it as well. On the reverse, 7 to 11 o'clock, onwards towards the center has black staining that appears to be environmental damage. At a coin show, this coin would easily blend in with the rest. If it was real, the value of it is said to be less than $100. But somebody took the time to try to make it more convincing anyway.
They said although this may not have fooled an experienced numismatist, it easily could have fooled many collectors out there. It was discovered to be a cast counterfeit by an NCS profession, exhibiting mushy details and porous surfaces that easily could be mistaken for environmental damage.
If somebody posted pics of that coin here, you would not be able to tell it was a fake by the average pic on a computer screen. The fake staining makes it even more convincing. It's a shame. Be on your guard out there.