"I often see people saying 'Nobody has produced an invulnerable software system; therefore, nobody will ever produce an invulnerable software system.… - Daniel J. Bernstein

Now he took the music of like Mandrill, like "Fencewalk", certain disco records that had funky percussion breaks like The Incredible Bongo Band when they came out with "Apache" and he just kept that beat going. It might be that certain part of the record that everybody waits for--they just let their inner self go and get wild. The next thing you know the singer comes back in and you'd be mad.

"So it's tempting to incorporate a smaller resolver library into qmail. [...] I'd no longer be able to blame the BIND authors and vendors for the fact that attackers can easily use DNS to steal mail." [From the file "THOUGHTS" of the qmail distribution]

Of course, the test difficulty depends on what you're doing, and on how you're doing it. I'm constantly asking "How much would I have to screw this up to write an incorrect function that passes these simple tests?" Occasionally the answer is "Not much," so I'll throw the code away and start over. It was probably perfect code, but that's not good enough.