Welcome to 2018/the modern world where all you have to do is complain and that person gets their way because it's the easiest method to kill the problem....Only ignoring the other parties side completely even if he/she is right. We live among adult babies. This is high school shit. Sad how the world has changed to this.

What was the behavior? Curious mind would like to know.

The building management's job is to take care of problems like this and it's better that they handle it than individuals confronting each other in the building. Technically, you may not have broken any rules but something you did annoyed somebody. I have lived in apt buildings for over 40 years and this is normal behaviour in a multi unit building. I would suggest that if this type of interaction bothers you a lot, you should probably move to a setting where you don't have any neighbours.

Let's play a game of "What about THIS- is THIS annoying?" What about cooking fish every single day? Throwing their garbage off the balcony into the dumpster? Dribbling water from a faucet so it makes a water hammer? Clomping around in high heels on wood floors at 5:30 in the morning before leaving for work?

There are plenty of things that are annoying as hell but not strictly against any rules. If something you do happens to bother someone the right thing to do is go "Hmmm they might have a point" rather than turn into a sassy pre-teen calling someone else a child.

maybe live and let live when stuff is none of your business.

Keep on doing it. If you're not playing loud music,or banging around then the problem is theirs,not yours.
Some guy complained I was parking my car nose (not breaking any rules,no where near him) out,instead of nose in. I kept on doing it despite letters from strata until he got so uptight and swung at me. I phoned the police and pressed charges.

Maybe you aren't breaking rules, but there are many ways to be an irritating neighbour and negatively affect someone's quality of life. Especially an upstairs' neighbour who wears hard soled shoes, uses appliances like blenders/food processors/garbarators at all hours, lets the dog bark when it hears an ant fart, and slams doors and cupboards closed on a regular basis.

So if you think the neighbour complaining to property management is inconsiderate, perhaps you should think about the impact your actions could be having on others.

I just had to deal with something like this recently. I have other people complain about noise from my apartment. Thing is I’m super quiet. I follow the house rules, no undue noise past 11. But my neighbours are old and have nothing important in their lives to deal with. One neighbour knocked on my door at night and told me my fan was too loud, it’s the middle of the goddamn summer. But like the original poster I “kept the peace” and turned the fan off. An hour later the maniac is banging on my wall screaming about the noise. So the board members had to be involved, they asked me if I would comply and turn off my fan and I told them I turned it off. They didn’t believe me. So I had to have one to stand in my apartment while another stood in my neighbours apartment and turn the fan off and on. Finally they believed me that I wasn’t making the noise, so they went off to hunt for the phantom noise that I don’t think they ever found. All this effort to appease my neighbour, even if the noise was coming from my fan it wouldn’t have been loud enough to have broken the house rules. You know what my neighbours do that bothers me? They read the house rules, say “I’m more important!” and then harass other tenants. Now I just won’t answer my door when someone knocks and will press charges if any of them go nuts and attack me.

When I moved into a new place a while back someone came pounding on my door demanding to know why I was taking a shower at 2 in the afternoon. I guess with people away at work the noise in the building during the day decreases so they could hear it more. They weren't worried about maybe there was a plumbing leak or something but that the sound of the building was different than their routine was used to. I'm not going to ask them permission to take a shower and made that clear. They haven't spoken to me since but they talk trash to other neighbors about me now.

This is so do depressing. @OP, if you're making noise or letting you're apartment door slam... Then maybe try to be considerate... What goes around comes around...