Spent a whole Sunday wondering why my enemy towers were dying by themselves all the time. They are towers that follow you along an island if you come near enough and shoot at you. So when they see you they begin to shoot and after some time just explode all by themselves.
It turns out they were killing themselves by the bullets they were shooting at you. The towers were designed as a body, a head and a cone twist constraint between these two. You can find more information about different type of constraints here.
With a cone twist constraint and me flying over their heads the towers were shooting sometimes right up. The bullets they are using are real objects with mass and form. So when a bullet is being shot up it falls down after some time thus killing the tower it came from.