Hashing functions such as MD5 are susceptible to collisions. That is, someone can take two documents and tailor them such that the resulting MD5 hashes are identical. One group showed this particularly well by hashing documents that shows who will win the US election. The group made twelve PDF documents all have the same MD5 fingerprint.

Predicting the winner of the 2008 US Presidential Elections using a Sony PlayStation 3

http://www.win.tue.nl/hashclash/Nostradamus/