This is one that has happened to me, and is worth mentioning - my D3 and Sigma 70-200 combination rolled off a chair onto a carpeted floor, and the lens broke off the camera. After the initial horror at what should be two exceptionally robust bits of kit seemingly being unable to survive such a small drop, I looked at the damage - both physical and financial.

 The lens had sheared off just ahead of the mount - none of the optical aspects of the lens had been damaged, and the damaged components were a plastic ring on which the lens mount sits, and a connector on the PCB. The cost of these components was under £50. Had the force been transmitted to a less-strong camera body, then there would almost certainly have been damage to the mount. Had the force been more of a jolt, it's possible the PCB would have escaped too.

 If this happens to you, don't panic. Your lens had just potentially saved you a very expensive repair bill, and would appear to be designed to do this. And whatever you do - be careful where you leave your camera!