Answer by Dreamblur

There is no key named "Jump." Your code applies a force of <0, jumpHt, 0> on the rigidbody during a single frame. Since the default ForceMode parameter of Rigidbody.AddForce() is ForceMode.Force, your rigidbody is basically receiving an initial upward velocity of 0.5 units/sec. That's a very small amount of force for a jump. Any upward velocity it provides is easily negated by gravity after the third frame. Adding both a character controller and a rigidbody to the same game object is just asking for buggy physics.