I knew a guy in high school who could sit and tuck his head all the way behind his thighs. In fact the yearbook editor used a photo of him in that position. So we all assumed he could accomplish the feat. But we never really witnessed it.
When I was in grade school there was a kid who could bend backwards so far he could put the BACKS of his knees on his shoulders! He could stand on all fours with his feet ahead of his hands.
That creeped me out something fierce!