I lived with my sister as a freshman. She's two years older than me. I ended up going to the same school as her, so the summer before my freshman year, my parents bought us a condo to live in. It was actually very cool because my she and I get along just fine.
The only bad roommate I ever had was last summer. I had just moved into my current place. One of my new roommates had sublet his room for the summer to this weird kid. He didn't have a fridge until I moved in, so until then he kept his food in an ice chest (pretty crafty of him). Every time I came home from work, the place reeked of pot. But the thing that got me: he had long since run out of toilet paper in his bathroom. I was in there one day (don't remember why), and what do I see? A stack of napkins sitting next to the toilet.
Needless to say, his stay in our house was cut very short. (Actually, the major reason I kicked him out was because he was actively selling out of our house. The napkin thing didn't help, though.)