It is a worst type of "bug" because it seems intentional: the implications are enormous, the temptation is simply too big.
Remember the "netscape" crypto lib (ssl huh...)? With half-key repositories, weak rnd generators, "bugs". This doesn't happen so often by accident. Particularly after having been recognized.
http://www.cs.berkeley.edu/~daw/papers/ddj-netscape.html
