I did step in a hole on Clay last year and sunk in up to my thigh. It was summer. I was very surprised as the ground just gave way and down my leg went. I bruised my knee and it took a few minutes to leverage myself out, but if I had been moving at any kind of speed instead of just drifting and looking around, I would have surely broken a leg or ankle. It was unlucky to fall into the hole, but damn lucky to not result in a serious injury.