Catch an edge.
Catching an edge on skis, much like on ice skates, leads to outside forces pulling on your extremities in terrible ways. This is especially true when going fast, and is especially easy to do in powder. Well gosh darn it, I should have known. It is also a particular threat when old injuries are still nagging, and is rather embarrassing when the little gaggle of nice people stopped to help REALLY want to call ski patrol. Well, we all know that would never fly.
Skiing bumps on one leg is also rather precarious and inconvenient, as is trying to carry a helmet and poles while doing so. Perhaps I have learned my lesson, at least until tomorrow when I am sure to do this whole thing all over again.