My freshman year roommate used to masturbate when he thought i was asleep and i had to cough really loud and move around so he knew i was awake to get him to stop.
I had a flatmate in Cairo that was OCD. If I left ANYTHING at all out of place he would duct-tape it to my door at night while I slept. He did this a couple times with my shoes and all the time with pens, notebooks, bowls of cereal I may have forgot in the sink, etc.
My second freshman-year roommate fell asleep listening (loudly) to the Lord of the Rings soundtracks on her computer speakers. Like, I love LOTR but waking up to the music from battle sequences at 4 a.m. is terrifying.
A girl we vaguely knew got kicked out by her landlord, so we let her stay at ours for a few days. On the first night, she screamed at me for opening a packet of crisps "the wrong way up," then tied bundles of herbs to all the doors for a reason she never fully explained but as far as we could tell was to keep witches out.
She didn't stay long.
One of my college roommates briefly dated a guy who seemed a little eccentric, but nice. One Saturday morning, she had to run an errand — he was hungover and decided to hang out in our living room until she came back. Not really a big deal. But eventually my boyfriend and I had to run out too, leaving him alone in the apartment.
I was pulling out of the parking lot when I realized I left my wallet on my desk. My boyfriend offered to run back in and grab it for me. When he opened the door to my bedroom, he found the guy crouching in the corner, next to my closet. Without any pants on. Wearing one of my scarves.
I had a former flatmate who spent all her rent money on laughing gas. That was... not particularly hilarious. Also, another former flatmate once brought a crab home from the seaside which escaped and went rogue in my house. Yeah, waking up at 6am to hear your other flatmate screaming because there's a crab on the staircase was an experience.
One flatmate kept pet white rats who would crawl all over him. Quite cute. Then they had a litter of babies. Even cuter. Except we went round after the pub one night to silence and a strange smell. Found him in his room, high on meth, surrounded by eight rat corpses. He'd broken their necks. One by one.