Not really nightmare, in terms of fighting or criminal activity, but anyway...
We had live in a townhome, and had some issues with one unit two doors down. The owner had moved out and rented her unit to a series of ne'er-do-wells. One set was a group of young people who threw late-night parties with 30 of their closest friends on their small deck several times a week. So, until the wee hours of the morning, there would be yelling and loud music. Not awesome when you have a newborn. We spoke with them one day, and the issue got better.
They moved out, and another moved in, and he was far, far worse. He was this very loud, brash individual who spent the majority of his time in his driveway on his phone speaking VERY LOUDLY to people on the other end. He had five dogs in his townhome, while any more than three is illegal in our city. They were all small loud yippee dogs. He would open the back door to his unit, let them all out, and them close the door on the leashes, leaving them all out there. They would bark incessantly for half an hour at a time. It was so loud. And he would never clean up their poop, so it stank like hell. One call to animal control and a few of his dogs disappeared, but he continued to let them out like that. He also put up ridiculous amounts of holiday decorations, invading his neighbors lawns as well. As it turns out, the owner of the unit had foreclosed on it while he was living there, and he fell into a loophole that meant he didn't have to abide by HOA rules (remember this).
But the funniest thing was the day before he left. I was at home one afternoon, and I hear some very loud yelling out front. I look out and see him in his driveway, freaking out, with water running down the driveway. Apparently, he had been ignoring a small leak on the third floor of his unit, and it finally burst, sending water pouring down all three floors and out the front door. When our courtesy officer tried to turn off the water, he refused to let him, thinking that his water would never get turned back on. They finally managed to stop the flow. When the property manager showed up, he started screaming at her, as if it's her fault. He demanded that the HOA insurance pay to repair his house and that the HOA put him and his family up in a hotel until it was fixed. Of course, the manager laughed in his face. He moved out the next day.
My only other experiences were my roommate in college, who made me swear off roommates forever. He left so much crap on the floor, I literally had to scrape out paths to walk across the room. He was loud and had a really annoying northeastern-mixed-with-florida accent. He would lay in bed for literally days after a Miami Dolphins loss. Drove me insane.