a couple years ago I was on a rock over looking part of the Black River in Missouri . . climbing it with the intention of jumping into a fairly deep part of the river (good fun).
I was wearing those river shoes . . mesh, really light, but enough to protect from rocks and the like. Anyway, they have those loops on the back . . . . . I went to move my right foot, and I couldn't, I looked down and a snake was in the process of going through my little hoop.
Couldn't ID the snake (fast ah heck), but we'd seen water mocassins in the area before, and lots of copperheads (though I doubt this one was a copperhead).