Sometimes roommates can just suck. I had a roommate that told me after the fact that he let someone stay in our place from craigslist. They thought the apartment looked cool for people looked cool even though we had a listing up for someone looking to rent another bed but he was only in town for the weekend. He didn't even know this person, they stayed there for a weekend and I found out on Monday.
RE: If there's one thing I've learned about roommates is that they always lie to you.