Any fan can do that not just flexes. (like I posted above) and is why checking it for stress cracks just as often as one would check the oil is a really good thing to do. (not that even the people that know that do it)
Probably seen it happed 4 times in my life and not one of those was a flex.
That rule of standing out of the line of fire from broken blades was around a long time before flex fans came out, and I think it originated in the hot rod circles.