I lived with a guy who'd steal my car while I was asleep. Once I learned of that, my keys stayed in my bedroom.
I had another bird who rented a room from me that decided it was cool to pour candle wax all over her bedroom carpet because she thought we weren't respecting her. We'd constantly ask her to clean up after herself and she wouldn't. My missus at the time, bless her soul, absolutely lost it on here and threatened to physically kick her out if she didn't leave at that moment. I was sitting there with a massive grin on my face while my missus tore a strip off of her something fierce.
Another nutter I lived with was a coke addict and threatened me with a shovel. He'd annoyed me so much that I goaded him on and said he shoud do it, because if he had actually tried I'd have beat him so badly he'd be coughing up his teeth. He saw the glint in my eye and realised that perhaps he should retreat.
The absolute worst however, was the most seemingly mundane thing... A roomie had his gf over all the time. As in she pretty much lived there - meaning she wasn't paying rent or utilities, but would take hour long showers, use the bathroom for extended periods of time and have ridiculously loud sex with her bf. Like, "Ooooh laaawd, Jeeeeesus! Fuck me, fuck me!" screaming at the top of her lungs. It was like going to a Southern Baptist church that had somehow started to worship porn. It got to the point where I'd see them retreat into his bedroom and I'd immediately crank up some very loud house music to counteract having to put up with their grunting and groaning.
Having lived with my fair share of people in my younger years, I can say that usually it's the mundane little things that usually add up to become a massive problem. I'm glad it's only my gf and I now. Now it's just me who pisses her off!