Its probably the companions sudden reaction to realising that you didn't mean to hit them.
Then they usually say "watch it, sister!" (or brother).
I think I might have heard that "you fight well" line, but it never occurred to me to keep fighting. I've accidently killed Vilkas couple of times, though - he wasn't levelled up and my character was pretty good with a greatsword, and then there was that time when I "sneezed", well, yol-toor-shul'd him in the face... Oops.