If you refer to the 2010 Flash Crash, while my memory is kind of hazy from that day, I suspect lots of folks tried dumping at a percentage loss (or had breakers in-place), but couldn't get a fill in that fast moving market. Market sells got filled who-knows-where on the way down. Speaking for myself, I rarely trade on the "first move" on anything downwards b/c I've learned first moves are usually wrong and I don't like getting whipsawed. Look at a chart of 5/6/2010 and how it reversed going into the close ... how angry would you feel knowing you'd ended up selling at the bottom?
That event caught everyone off-guard, as memory serves...and Ben's audio from the pit is a trip down memory lane. IMO bottom line, sometimes the best thing to do is hold pat and ride it out until you know what's happening ... like turbulence in a plane, yes it's scary, but the only way out is through, and I didn't do a darn thing that day in any of my trading or investing accounts. And if you lose some $$ in the process, that's part of the game, as the other poster said. (and a tax harvesting tool!)