I had an awful roommate this past year. There wasn't one major thing he did, but all the small stuff added up. Always coming into the room late at night, not being quiet when he did, talking loudly on the phone in the room instead of leaving to let the rest of us watch Psych in peace, etc.
Freshman year, I had just moved into the 4th floor of Appelt. My potluck roommate comes in, and the first night we are in the room together, I (1) sleep with my eyes open, (2) yell the freshman wildcat in my sleep, and (3) hiss in my sleep . . . needless to say he was dreading living with me. We ended up being great friends and roommates still to this day (Senior year, whoop!)