I jacked a visitor's badge from the Eisenhower Executive Office Building in Washington, DC, last month. The badges are electronic; they're enabled when you check in at building security. You're supposed to wear it on a chain around your neck at all times and drop it through a slot when you leave.
I kept the badge. I used my body as a shield, and the chain made a satisfying noise when it hit bottom. The guard let me through the gate.
The person after me had problems, though. Some part of the system knew something was wrong, and wouldn't let her out. Eventually, the guard had to manually override something.
My point in telling this story is not to demonstrate how I beat the EEOB's security -- I'm sure the badge was quickly deactivated and showed up in some missing-badge log next to my name -- but to illustrate how security vulnerabilities can result from human/machine trust failures. Something went wrong between when I went through the gate and when the person after me did. The system knew it but couldn't adequately explain it to the guards. The guards knew it but didn't know the details. Because the failure occurred when the person after me tried to leave the building, they assumed she was the problem. And when they cleared her of wrongdoing, they blamed the system.
Really intersting read.