I've seen it happen a handful of times in our Fleet racing. Biggest cause (besides capsize) would seem to be trap gear failure or accidental unhooking. Another time it was a well-intentioned effort gone wrong by one boat to drop off the crew to help another boat that had capsized. The last one was a missunderstanding of how fast our boats can move even when capsized, leaving the crew that was intended to help in need of picking up.

In all of the cases except the last one mentioned, the boats were handled effectively enough to retrieve the MOB (including a case where a very young son single-handed a N5.8NA in decent wind to retrieve his Dad who had fallen off). In the botched assistance case I picked up the stranded crew.

Once my crew rolled off the lee foredeck in light conditions - he was able to grab hold of the boat before he got separated. Another time one of my crew lost the hook just as he was heading out on the trap - MOB drill, picked him up, continued the race...