A few years ago a friend of mine and his girlfriend agreed to board his parents' cat at their place while the parents were on vacation. The friend's cat, who was still fairly young at the time and had never met a fellow feline, was excited to have another cat visiting her house. However, the friend's cat and the friend's parents' cat just did not get along. Despite the friend's cat's attempts to make nice with the other cat, the other cat was still hostile toward her.
In fact, so hostile was my friend's parents' cat toward his cat that sometime after my friend and his girlfriend had the mean cat at their place, my friend, while grooming his cat one day, found a claw in her head.