Tailgating 

Tailgating (also known as piggybacking) is a form of physical social engineering. Tailgating can be defined as a physical security breach where an unauthorized person follows an authorized person into a secure area. 

A common type of tailgating would be someone waiting around a common area with their hands full for an authorized person to open an access-controlled door. During this time, the unauthorized person could ask them to hold the door open while they rush through. Some other forms might include striking up conversations with employees at a common smoking area. By the time the employee has completed smoking, he or she will likely hold the door open for you, masquerading as an employee. Humans have common courtesy, which can lead to vulnerabilities, such as holding doors open for unauthorized people.

Some organizations have good physical security in place, so this might not work everywhere. However, performing sufficient information gathering on the target's physical security will help you plan your attack.

As you perform penetration testing, you can leverage any of the preceding techniques within your penetration test. Having a good background understanding of what each technique entails will help you plan your penetration test more effectively.