Not exactly a "bad neighbor" story, but it definitely affected our neighborly dynamic.
Used to live in the upstairs of a duplex, that was just a modified split level house. My roommate and I had a friendly relationship with the two girls who lived below us. We would always be cordial and would hang out on the front steps and smoke cigarettes and maybe have a drink once in a while.
One Saturday, my vehicle was in the shop, and so the dealership dropped me off at my house while they worked on it, and I was going to get a ride to pick it up when my roommate got home from work. The reason I mention this is because no car was in the driveway, you might assume nobody was home.
That's what one of the downstairs neighbors must have thought when she got home. I was playing a game on the computer, when I heard her pull up and enter her apartment.
I didn't think much of it until about 10 minutes later when the sounds of some very vigorous and enthusiastic "self love" started emmenating from the floor below me. The sounds went on for a while and got a lot louder. I wasn't sure what to do, stay quiet or alert her to my presence. Fate, however, didn't require me to decide, because my phone began to ring. It was my sister, of all people. I picked up, and immediately the sounds from downstairs stopped, as my neighbor became aware that there was someone else home. I made chit chat with my sister, and heard the front door open and close followed by the ignition and departure of a car.
I never knew for sure, which of the downstairs girls it was that day, but I have a sneaking suspicion it was the one who never made eye contact again, or smoked cigarettes on the steps anymore.