A couple of walkers I have seen. This first one was from a sounder and the shot was my buddy's opening shot. The hog went down and bounced up and my buddy picked it up as it went by.
This next hog, somebody put a shot clean through behind the ears, tearing up the right ear in the process. It was mostly healed by the time I put it down.
If the shot doesn't actually do sufficient CNS damage...
I don't put 2nd shots into hogs as a matter of practice, but did for a while. Now I only do it if I think they are getting back up.