Insecurity
Road system in physical world is designed for safe (secure) use - sign board, speed limit, road shoulder, proper lane separation.
There is occasion insecurity taking place. There are many contributing factors such as:
Adverse weather (low visibility, slippy road, hurricane)
Malfunctioned equipment (vehicle)
Collateral damage due to other road accidents
Body condition of driver, under medical or drug influence
Inexperienced or negligence drivers
Similar principles apply in cyber world
Untrained user or human error
Failure to handle exception situation properly
Unpatched system components exposing to known vulnerabilities
Attack from peers nodes of connected system
There is one more contributing factor: if security hasn't been integrated into design and deployment of the target system, it won't be secure....