> But the difficulty of writing secure software is not about avoiding any one particular mistake; it's the sheer number of mistakes available to be made, so that it's easy for one mistake to slip through the cracks.

I'm not well versed in security, so I can't say anything about this particular mistake; but, as a mathematics teacher, I can say that this is a fantastic description of the perils inherent in any creative activity, not just programming. I hope that I have your permission to steal it for my own future use. :-)