What DIDN'T give it away? The paper was too thick and the wrong texture--kind of like oily construction paper. It was a uniform light green, like a child's drawing of money, instead of that sort of white-with-a-touch-of-green that older bills should be. Held up to the light, the watermark appeared as a black portrait, instead of as a lighter, blurrier version of the bill's actual picture. One of the bills did not have a security strip even though it purported to be from 2001. The ink on Franklin's jacket was not raised, and the colour-changing "100" in the bottom right corner was broken (the ink there is usually thick, richly metallic, and glossy--this was scanty) and did not change colour. And while I can't say, exactly, "this is what was wrong with Franklin's face," there was an indeterminate "I can't put my finger on it, but I handle hundreds of these bills and something is wrong" about the shape of the nose and the hairline (one of my coworker's calls this "banker's sixth sense"). One of our coworkers also commented that something was wrong with the serial number. I would call that "wannabe witness syndrome" ("Yes, officer, now that you mention it was a horrific triple murder I DID hear that gunshot"), except that she is also a currency collector and while neither of us specialize in bills, she's levelheaded enough that when she says "something was wrong with the serial number," I believe her.
There's also what I call the "wrong factor." You know that funny feeling you get in your stomach that some people call 'gut instinct' and some call a 'hunch'? I knew before I ever touched the bill, and no magic pen was going to tell me otherwise, that the watermark looking back at me was going to be the fake black one. I described the feeling to a friend later and she said it was "the uncanny valley"--the bill looked just off enough to give me that queasy, kind of disgusted feeling. She got it absolutely right.
"It is my belief . . . that the truth is generally preferable to lies."
"Differences of habit and language are nothing at all if our aims are identical and our hearts are open."
--Albus Dumbledore