Sadly, I think this problem is even more insidious with computers / programming. This is why most software and hardware companies have people whom they pay to do nothing but testing, and pay well. With an application of any complexity at all, the parts interconnect in such ways that it is impossible for one person to grasp and remember them all. Of course, we all do incremental testing, but there are always tests that don't get done, situations that don't get tested. I think it's the nature of the beast, and it's what keeps the whole thing interesting.