When going to college the first time (before dropping out), I lived at an apartment complex that was originally built to be a dormitory. You share a kitchen, bathroom, and half-bath with up to 3 other people. Rent was way cheap in a town where everything else is way expensive. So I get in a quad with, let's call him Ted.
I was way socially oblivious at this time. Ted is nice, doesn't bother anyone, he just stays up late and smokes and paces in front of the building and drinks lots of Diet Pepsi. He is clean, and never bothers me about noise, so we coexist with no difficulties.
It never occurs to me that maybe Ted never sleeps, or hardly ever. After living beside the guy for almost a year, I actually did sit and have a meal with him once, he talked about being a psych major, we talked about parent issues, in his case, issues with his mother.
2-3 months later, I hear a lot of yelling from Ted's room. Lots. The guy hardly ever spoke above a whisper since I've known him. He sounds upset, enraged. Sometime in the next couple of days he is gone, moved out.
About a month later, one of the maintenance guys on the property asks me how I got along with Ted. 'Fine,' I say, 'he seemed great, no problems'. He gives me a funny look. 'Why, I ask.' The guy says, 'He was crazy, you know. Thought the devil visited him every night. None of his other roommates could stand him.' I thought, THANKS, ASSHOLES, for fucking TELLING ME!! But, here I am, not dead.
Later on, when my girlfriend moved in with me, we rented that room. I dunno if it was me or leftover Ted-vibes or what, but that room did give me some odd gut-feelings for awhile.