I’ve been having this problem for a couple of weeks now; it still occurs in 0.9-2784-g0b8f3e6. It seems to only happen when the knockback blow kills the creature; non-fatal knockbacks seem to be ok.
I’ve got other symptoms as well, though. The square where the creature was previously standing becomes bugged for a while. Attempting to move into it causes you to take a swing at some other enemy, perhaps the next-closest one. You can damage and even kill a creature that’s not actually there, and it can’t hit you back. If you kill the creature in the bugged square and continue to swing, you’ll hit yet another creature. Enemies will avoid the spot while it’s bugged, too. If you’re fighting a good-sized horde and inflicting a lot of knockback kills, it gets hard to find a non-buggy path. The problem clears up after a little while, though I haven’t figured out specifically what un-bugs the square.
Edit: Whoops, this is this issue.