OnCollisionEnter event is not working when camera (FPS Character Control / Capsule Collider) collides an object
99.9% of the cases, you missed out the Rigidbody requirement.
Straight from the docs:
Note that collision events are only sent if one of the colliders also has a non-kinematic rigid body attached.
I have solved this problem myself by creating an external capsule collider (A transparent capsual having no rigidbody containing mesh collider).
[17628-3d+maze+game.jpg|17628]
It is working properly
