A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools. ~ Douglas Adams
@AskNick It's true. When you aren't a fool, it is very hard to think like one.
There was a manager at one of the places I worked that was as dumb as a bag of hammers, but we loved having him test interfaces.
If there was a way to generate an error, he would find it!
He also approved of our cannabis testing of application interfaces - testing whether the interface can be used by someone who is high.
(That's actually a useful methodology, BTW)
@AskTheDevil We had two guys we'd give new stuff to. One was quite brilliant, and found great flaws. The other found flaws of the "why would any sane human being do that?" variety.