It HAS happened to me, I have also been present when it happened to two other divers. At the time I was overweighted, but felt it slip & grabbed it in time. The other two times, luckily the divers in question were not overweighted, , so they weren't carrying a lot of weight.
When diving single Al tanks in fresh water, I wear 3 lbs on my belt. I wear my belt UNDER the crotch strap. If I or someone were to need to dump my weights, our hand would already be on the quick release buckle to unlatch it, so just how much more time would it take to pull the belt off of me?
BTW, I would say that getting pinned against the ceiling of a cave would be better than doing an uncontrolled ascent in OW from depth, so I would be even more worried about there.