Every so often a story gets going about someone having an animal go down, get back up, shoot it again and ending up with 2 or more dead animals. I've never known of a VERIFIED case of it happening but it still pays to make sure you're always shooting at the same animal.
I'm sure that's happened to many people. My elk hunting partner's old partner was known for doing this. There's also the story of the guy that shoots a bull and the one behind it falls down too.
. Moral of the story, know your target and what's behind it...