Tailgating
Tailgating, in access control, is a second person passing through a controlled door on the credential of the first, so the badge log records one entry while two people went in. It is the most common way a secure entrance is defeated, and the badge reader cannot see it.
Sometimes it is courtesy, holding the door for a colleague; sometimes it is deliberate. Either way the access system's record is wrong, which matters in a data center, a server room, a pharmacy or anywhere the log is the audit trail.
A camera on the door can count. Tailgating detection tracks the people passing through the doorway per access event and raises an alert with the frame when more than one goes through on a single badge.
On Camzify it is licensed as an instance per camera on the doors that matter, and the alert carries the frame and the time so the access log can be corrected and the person told.