This subject came up on a cycling blog thread the other day. It’s worth looking into. It’s never happened to me (now that I’ve said that it will), but I’ve been with people to whom it has happened. It’s absolutely terrifying and they were understandably rattled. And although I’ve never been with anyone who has gone down because of it, it often results in a crash.
This is a really good article on the subject and explains how and why they happen. Knowing the physics behind them really helps to understand how you might bring them under control and not crash. And, as we all know, knowing is half the battle!