This seems a good time for comfy storytelling, so let me regale you all with the worst mistake I ever made DMing.
To set the stage I was pretty new to DMing, and the party (a rouge, fighter, druid, and barbarian) had just hit level three. I figured it would be a good time to set up a session where someone could get a proper magic weapon. My brilliant idea on how to do this was to split the party based on alignment, and have them fight over it.
They traveled through a forest on their way to the next town they have business in for their epic quest. They are traveling as guards for a caravan. A grey stone road divides the forest and provides an easy trail. On the right hand side of the road the forest is green and orderly. However the left hand side is dark and tangled. The caravan camps for the night in the forest, and strange sounds and lights are noticed coming from the woods. The party is sent out to investigate and warned not to stray off the path.
They see will-o-wisps and faerie dragons fighting, anyone who steps off the path gets attacked according to their alignment (faerie dragons attacking evil, wisp attacking good, neutrals being attacked by both) After a couple of rounds of battling a Unicorn and Nightmare (homebrewed like an evil unicorn) show up and call a halt to the combat. They each control one side of the forest and battle each other for supremacy. They ask the party to pick sides, each arguing for why their vision of how the forest should be is correct (SMT law vs order style).
I was confident I could get the evil rouge and the neutral (easily bribed) barbarian on the Nightmares side, and the fighter and druid, who were both good, would join the unicorn and have a fun brawl the next day at dawn. Unfortunately the druid decided not to get involved in the mess. Then the Barbarian backed out at the last second, after I had given the fighter a couple faerie dragons to even things out. So I tossed the rouge an equal number of wisps to even it out.
So with it being more 1v1 things got more personal than I had planned. The fight ended up very close with the Nightmare dying first. The rouge could have finished the Unicorn but rolled minimum damage and it teleported away. What I had thought was so clever was that the person who lost the fight would be the one who got the magic weapon. The Nightmare turned its horn into rapier for the rouge to remember him by, while the fighter only got some healing potions and gems.
So instead of a nice consolation prize the rapier kind of became a memento of the time the two characters tried to kill each other. So yeah, be careful having players fight each other, and never trust them to act according to their alignment, rookie mistakes.