(wall of text warning: not much drama though)
The situation was, my roommates and I (four total) needed to add one more on to the lease as a subleaser, after one roommate moved back home. The lease had already been signed, and to add on a subleaser would require everyone (the four currently in the house, the one that moved out, and the one that wanted to move in) present. This proved extraordinarily difficult because everyone had totally different work/school schedules and the one that moved out wasn't even in the same town. In the end we just said fuck it, you can live here unofficially, and we were all happy. I included this for context because in retrospect, I believe this roommate thought because he wasn't formally on the lease, he could get away with the things he did.
Everyone was super happy and cool the first four months he moved in. Quiet guy, spent a lot of time in his room, but was polite and paid rent. I was in charge of paying utilities, and at the end of every month I told my roommates the total of the utilities that month, and they'd pay me their share. Everyone was happy.
One month I made a stupid error where I included the previous month AND the current month of electricity in the total. This bumped the cost per person up about $15 past where it normally was. The new roommate noticed this and asked me why the cost was so high. I discovered my mistake after checking my math and thanked him for pointing it out, everyone paid, everyone was happy.
After this, the roommate got quieter, and I thought maybe he was going through a rough time. One night, though, he sat down at the table I was working on and demanded an apology from me for trying to screw him over and cheat him out of money. He thought my stupid math error was intentional, and from that, he deduced that I had been lying to him and all my other roommates about bills the entire time we'd been living together. It was inconceivable to him that I could have made a mistake. He said a lot of horrible things to me, called me the lowest human being he'd ever met, etc. It was silly.
After that, I guess he just decided that to pay me back for my "scam" he didn't have to pay for anything. Stopped talking to everyone in the house, and barely even came downstairs except to eat and pee (in the middle of the night when no one could see him). It was kinda frustrating because rent still had to be paid, and that money ended up coming out of everyone else's pockets. He refused all forms of contact, no matter how many times we told him to pay the goddamn rent. After about a month of this, I finally cornered him in the kitchen and had a one-sided discussion, telling him that what he was doing was bullshit, we wanted him out, and if he did not voluntarily move out, the police and apartment manager would be more than happy to help. I told him he was scamming the rest of us far worse than he thought I had to him, and that not being on the lease did not give him any rights to skip out of his payments. He had no answer, so nothing got resolved.
About a week later, he came downstairs to meekly apologize for not paying rent, and thank me for my patience and understanding in having him stay without rent. He explained that his car had broken down, and as a result he was forced to quit his job and drop his classes. (Which is bullshit because we have a very good bus system that runs right from our apartment to campus/his work). It did explain why he was spending 100% of his time holed up in his room, but it did not explain how he was unable to pay rent but still able to sit up in his room getting high 5 times a week. He promised his parents were loaning him money so he could pay me back "soon", and I told him I didn't believe him and to hurry and move out. Two days later, we woke up and all his stuff was gone. Never heard from him again. It was awesome.