At least three times (twice with the Mk I and once with my brand-new-at-the-time Mk II), my 7D body unexpectedly came off the lens to which it was mounted and smacked the concrete runway (an airport racetrack) hard. Each time, damage was only superficial and minor (some light scratching on a corner). Turned out that the cause was the Blackrapid dual-strap system, from which the 70 - 200 lens and 7D were hanging from, was on my right-hand-side and the camera body would hit my side in just the right way so as to push that button that releases the lens. Solution: switch the 7D camera to the left side. I haven't had a recurrence since changing sides.
That's the one complaint I have with BR, the camera/lens has a tendency to swing around or bump into my leg.
I had a lens hood come off, and the switches moved on my lens due to this bumping against my leg.
One must be careful.