The knapsack ciphers were an example of a great sounding idea that failed catestrophicly. It went from being highly secure to totally broken in a short period.
<http://dsns.csie.nctu.edu.tw/research/crypto/HTML/PDF/C82/279.PDF>
<http://www.rsasecurity.com/rsalabs/staff/bios/bkaliski/publications/other/kaliski-liskov-tale-of-two-asiacrypt-1998.ppt>
<http://www.cecs.uci.edu/~mingl/knapsack.html>
<http://www.google.com/search?q=cache:oGjwvE9HUUgJ:www.bhargav.com/books/Computers/Applied_Cryptography/appliedcrypto/ch19/19-02.html+merkle+broken+stage+knapsack&hl=en&client=firefox-a>