If a roller coaster got stuck upside down, what stops the pull down harnesses from popping open?

Just wondering. I was on tatsu at six flags L.A. a little while back and right before we started to go, all the harnesses popped open and they had to go back and close all of them again and make sure they were working. I was just wondering if that could ever happen while on the ride or if you got stuck upside down. lol.