A Pegasus bone's must be indestructible. She's crashed into mountains, walls, through buildings, survived being a Mach force projectile when launched back while attempting a sonic rainboom, crashed ribs first into Twilight's balcony, and just generally takes a beating, and she is never really injured.
Twilight Sparkle falls down a flight of stairs, and ends up in a wheel chair, bandaged in several areas (though apparently she can heal up immediately)
I just find it funny, seeing as one of the main fanfic ideas people run with is "Rainbow Dash breaks her wings". >_<
My cousin shattered her heel once. That's one of the worst places to shatter. They had to get a specialist in to do the operation. She still suffers from the ramifications of that. So I hope RD didn't shatter her heel, because I would be the nit picky fan saying, "She can't heel so easily from that!!" >_<