Once I shoveled some stairs immediately after the first snowfall of the year, on a night well below freezing, and there was a boot print of solid ice frozen onto one of the steps. It's one of the creepiest unexplainable things I've ever seen. I can't think of any way it could have formed, and it was in the middle of a staircase with at least a dozen steps.

My best guess is that, because it was a wooden step, the boot print was permanently imprinted into the step itself, and somehow it had filled with water and frozen before the snowfall.