In general, people are some of the weakest points of any system. Today, Mark Gazit is CEO of ThetaRay, a leading provider of big data analytics solutions, but in the '90s, he worked as a penetration tester, and a large bank offered a large bonus if he and his team could break into a system that was believed to be unhackable. "We tried to hack it from the outside, but could not," he said. "However, we learned that on the bank's premises, there was a raised access floor under the computer room, under which lay computer cables and a way to sneak in from beneath." Did this call for some Mission: Impossible-style derring-do? Well, not exactly. "I had one of my guys tell the guard that somebody was looking for him. He didn't just leave his post; he gave us the keys to the computer room, telling us, 'If anybody needs to enter, just make sure they sign in.'"
A system isn't unhackable unless all its components are unhackable -- even the components that walk on two legs.