They do that an infinite number of times in practice, often on purpose when they’re learning a release. That’s one of the safest falls you’ll ever see in gymnastics.
Olympic gymnast Shannon Miller says you should NEVER stick your arms out when falling, hence the belly flop:
You should never try to stop your fall or brace yourself by sticking your arm out. I learned this the hard way when I reached out my hands during a wayward uneven bar dismount, dislocating and breaking my left elbow just 10 weeks prior to the 1992 Olympic Trials. After that I never once tried to break falls with my hands.