Hear I and Eric how bad entropy caused some limited problems in the bitcoin block chain:
https://plus.google.com/u/0/106313804833283549032/posts/X1TvcxNhMWz
From a linked page: “Why did this work? ECDSA requires a random number for each signature. If this random number is ever used twice with the same private key it can be recovered. This transaction was generated by a hardware bitcoin wallet using a pseudo-random number generator that was returning the same “random” number every time.”