Someone showed me this years ago, and it really is quite scary.
If you take a 5, 10, 20 and 50 dollar bill, and fold them in exactly the same way, you get frightening representations of iconic U.S. Government buildings and the trees that surround them, being turned into images of the attack on the twin towers on September 11, 2001.
Note that although these bills have been reissued from time to time with enhanced security, they have not been fundamentally redesigned since well before the tragedy of the twin towers in New York.
Starting with the 5 dollar bill. The back of the 5 has a picture of the Lincoln Memorial on the back, and was originally issues in pretty much this form in 1929. Usually it looks like this:
However, if you fold it in half, and the fold the two sides back on themselves, it looks like the twi towers, complete with antennas.
Next, the 10 dollar bill. This was redesigned in its current form in 2000, and features the U.S Treasury building on its reverse:
Fold it exactly the same way as above with the 5, and you see this image:
The 20 dollar bill was designed in its current form in 1998, and shows a picture of the White House on its reverse:
Once again, fold it in exactly the same way, you see this image:
Finally, the 50 dollar bill shows a picture of the U.S. Capitol on the reverse, with the note being redesigned in its current form in 1997.
Fold it the same way as the others, and you get this eerie ghosted image of smoke rising where buildings used to be.
And if you line the all up alongside one another you can see just how scary this is!
Conspiracy theories about the twin towers attack abound. Click here for the wikipedia entry dedicated to the various theories.