Gizmo and my cats have something of an ongoing sibling rivalry. No real aggression, of course, just an ongoing pattern of annoying each other.
This morning, one of our cats, Taz, was sitting with her back to the bars of Gizmo's indoor play yard. I watched as he hunkered down, very carefully crept over to the bars, and swatted her one. She responded by turning around and batting him in the nose about six times.
Gizmo's answer to this was to come limping up to me, whining pitifully. No doubt saying, "Mommy! The bad kitty was being mean to me!". It's like it never once occurs to him that I SEE him starting it. The limp was a nice touch too--I had no idea that a well-deserved swat on the nose could make you limp.