I was GM of a Cthulhu D20 game (d20 version of call of cthulhu, we were more familiar with those rules) and suffice to say, it went surprisingly unwell.
I had one player who was a samurai with a katana (I figured hell, why not, could be good for laughs and will keep him interested.)
So, my players are in a warehouse, killing rats (I don't know what else to start with.) The rats are in a vent system that's hanging from the ceiling, so my player who is a journalist gets out his grappling hook, and manages to catch the vent, and then helps my samurai player climb up there. So, he gets up on it, and start stabbing downward into the vent, attempting to kill the rats. The journalist player gets the brilliant idea for him to cut the vent from the ceiling (it's cheap metal) so he tries. He digs his katana into the vent and starts running, and cutting the vent in half as he goes, lengthwise. About 3/4 of the way in, he hears a bloodcurdling scream, and the other players see a large amount of blood running from inside the vent. The vent then splits in half, and dumps the remaining rats out, followed by the corpse of a human child, that is now missing an arm. He hits the ground with a wet thud; my third player (I believe he was an investigator) fails his sanity check and starts screaming, and falls to the ground in the fetal position. Samurai player climbs down the brick wall, and rolls the child onto his back. He has a small blanket tied around his neck as a cape, and a burger king crown that says 'Rat King' on it in sharpie. Journalist player tries to calm down the Investigator simply by saying 'He was a burden to society, and he had dementia. He's better off dead.' They stuff his corpse in a bag of flower, finish off the rats, take their money and leave. This left me in a bad spot; the kid was the link to the next mission, they were supposed to find his parents, and then would figure out that there have been large amounts of missing children lately..so instead I had to pull something out of my ass.