Happened to me once. I was super drunk walking home and didn't see an open manhole in front of me. I got super lucky, though.
From my drunk perspective, I'm just walking along when suddenly the ground is nearly at my eye level. Then I realised I'm dangling there, with only my head and elbows outside. I dragged myself out and continued on home.
I have no idea how I managed to fall inside with both my legs at the same time and why my arms didn't hurt like hell, not even in the morning.
I don't think reflexes were involved here, more likely it was lucky arm positioning at the right time. But what do I know? I wasn't quite there to witness it.