People are the weak link in most security systems. This much is said frequently by security experts. The BBC has published an article that introduces the business of physical penetration testing, a task carried out by tiger teams. The idea is that you pay a company to attempt to gain physical access to your building.
In this example, Colin Greenlees, a consultant of Siemens Enterprise Communications, tells how he gained access to a publically-listed financial company by fooling the MD to hold open the door for him. He then set up camp in an empty office for five days, inviting a collaborator to join him.