For me, my worst roommate experience was about, 4 months ago
Some guy moved in for about a month for some reason, and so I gave him the other half of my dorm, cause I had a double to myself. After a week, the entire floor was just COVERED in diet soda bottles, old clothing, a bong or two, skateboard parts, bags of chips, old food, etc. It was impossible to step on the floor, because you would end up stepping on some trash he threw on the floor. Also It smelled HORRIBLE. I just drew a line down the middle of the floor, and shoved his stuff over. The RA of the floor was almost considering kicking him out of the dorm early, because the room probably broke like 25 federal health laws. He also somehow managed to go broke in the 4 weeks he was there. He had spent over 4 THOUSAND dollars in a 4 week stay. And his parents KEPT giving him money when he ran out. He also came to the dorm at night smelling of drugs, which I can't stand.
When he moved out, I had clean my room for hours trying to get the gunk off his side. We also had a slight insect problem, something which my side of the dorm never had. Worst experience ever.