I've roomed with some pretty cool people, but I've also roomed with total nuts. I have a temporary roommate right now (who's moving out this month), and we get along pretty well--we've been friends for years, though. But now it's time for horror stories.
-----
One of the worst roommates I ever had was an alcoholic sociopath with an obsessive-compulsive disorder and a crazy girlfriend who was over at our place all the time. I'd come home from work each night to either loud sex or screaming matches, sometimes both. I remember one fight my roommate and I had that began like this (this is pretty much verbatim--I am not making this up):
Roommate: "Could we have a talk? There's something I want to get off my chest."
Me (sitting down at kitchen table): "Okay, what?"
Roommate: "Well... well. Back in November [this conversation took place in February, three months later], when you did your laundry? You left lint in the machine. A little clump of lint in the machine. And I didn't like that."
Me (after a pause): "Oh--sorry. If you don't like the way that I treat my washing machine that I paid $500 for, then just don't use it anymore."
Roommate (screaming): "I DO NOT LIKE YOUR TONE OF VOICE! I DO NOT LIKE IT!"
We had conversations like that every couple of weeks or so. Another time we had a fight because he came downstairs and saw me walking on the kitchen floor. He had just finished cleaning the floor, using a toothbrush for sections of it. "How else do you expect me to get around the kitchen?" I said. "The floor is useful when you want to get from one place in the kitchen to another."
-----
I had another roommate once who, when he moved in, said, "Is this place quiet? I don't like too much noise." I said I was never there much anyway, and when I was, I'd listen to the stereo with headphones if he wanted to get to sleep. His definition of noise was a little more strict than mine, though. After a week, we had this conversation:
Roommate: "The radiator in my bedroom is making a lot of noise."
Me: "Well, like what? Are the pipes banging?"
Roommate: "No... it's just a hissing sound. Like--sssss. I am lying in my bed, trying to get to sleep, and then the radiator goes sssss and it wakes me up and I am startled."
Me: "Well, all radiators do that. You'll have to live with it."
Roommate: "This is impacting my quality of life, and it's a violation of the lease." (I made him sign the lease along with me, since I'd been burned in the past when I didn't make everyone who lived with me sign a lease before they moved in.)
Anyway--the roommate called the landlord practically every day for a month until the landlord sent someone by to actually disconnect the radiator in his bedroom. And he still complained about the noise. I said, "Look. The radiator is not actually attached to a pipe. It is a freestanding object that is no longer working." The roommate said, "It is still making the hissing noise. I don't know how. Also the mattress I have is giving me an illness. This is a violation of the lease and I want out." I said, "Okay--I'll see you in court," and he shut up after that. We barely spoke to each other after that until he moved out, though.
-----
Words of advice for selecting roommates--
--You don't necessarily want someone cool, or someone to hang out with on the weekends. The perfect roommate is someone who gets up early, goes to work, stays there all day, and then comes home and goes straight to bed. They may have a crappy life, but they're great to live with. I've had roommates that I'd go without seeing for days at a time. It was like I lived alone, but at half the rent.
--If your roommate does something that gets on your nerves, say something right then, the first time he does it. Don't let it wait--otherwise it'll become a habit.
--Roommates get along better when they have roughly the same income, because then they'll have similar tastes and expenses. An unequal income means that one roommate (who, in this example, is me) will have nice things, like a big-screen TV and a nice DVD collection, and the other roommate (who, in this example, is one of my crazy-ass roommates) will use your nice things constantly without bringing anything to the apartment in return.
--Graduate students can be some of the worst roommates you can come across, and they're often the ones who can't afford places for themselves. Nothing brings out craziness in people like an unstructured but stressful academic life.
Jesus, I love living alone.