never so far, on bigger critters and i hope to keep it that way. lots of times with a machete on nasty aggressive snakes.
only things i run into is coyotes, wild dogs, occasional black bear, gators, snapping turtles, wildcats, cotton mouth snakes, eastern rattlers, pygmy rattlers, coral snakes....that's about it, but a knife is not so great on any of them. i'd prefer a big long stick on most of those, to chase 'em away, other than the black bear, and coyote for them i prefer a firearm.
most run the other direction when i run into them. which is a good thing, imho. one that scares me the most are those cotton mouths. lots of them around here, and if you are between the water and said snake, it will come right at you to get to that water, when you spook it. only critter other than humanized(fed by morons) ones i've ever ran into that will come right at you to get where it wants to go to feel safe. that's why i carry a big machete. first i try to jump and get out of the way, but i've had a couple come at me and try to strike. i'm all for live and let live stuff unless hunting for food.....but i just don't like snakes. never have and if they don't scatter i put 'em down, and i don't feel bad about it either.
edit part...for any brave kids reading this site and this post......these snakes are lightning fast and poisonous. do not go out and try to behead snakes for fun....it would be really dangerous and stupid. it takes allot of practice to hit one without getting bit, and they can jump pretty far. the smart thing to do is stay away.