When I was about third grade, back when neighbourhood children used to gather in large groups and play, the most vivid unexplained experience that has ever happened to me occurred.
I and my best friends liked climbing the trees in a small park close to where we lived. We especially loved one mulberry tree, because it was easy to climb, and its fruits were accessible. That tree was at the end of the park. Just about six meters away from it was an old house, in very poor condition. It was one of those houses that look like they're out of a Grimm brothers' fairy tale. It had two stone gargoyles above the windows, and a very creepy woman's head, also made of stone, between them. Its windows were mostly smashed, but the holes were too small for anyone to pass through without having to cut themselves badly, and break more off the glass. We were always curious about the house and talked about daring each other to go in, but none of us were brave enough.
One day it started raining, and the only place that was dry was the house. It wasn't raining above it or its yard. We were about ten or twelve people, all about my age, and when somebody proposed to go in there, nobody really wanted to be a coward. Besides, we had been waiting for a reason to go there, and we had an opportunity to justify not going alone, so we went in. We started circling the small yard, and peeking inside. I was at the front door, looking through the window next to it. There was a rusted old bath tub, a wardrobe, a coat hanger and the rest wasn't visible, apart from the stairway that went up. I remember it clear as day.
One of our friends started wondering how we could get in. She was looking for something to pick the lock with, and after failing with several items, she shook the door by the handle violently, out of frustration. Immediately afterwards, there was a sound like something huge fell on the upper floor, then the ground floor less than a second later, and finally, just as quickly, something heavy threw itself or fell against the door. We went out as fast as we could, and after a few hundred meters of running, we stopped and started talking about it. I didn't see it myself, but my best friend and the girl who tried to pick the lock swear, until this day, that there was a man on the second floor window, who seemed pale and was looking straight at them.
We asked the people living next to the house if there was anyone living inside, but they said there's only a young man who comes once in a while, about twice per year or less, and he had been there just a couple months prior, so that definitely wasn't him. They were positive nobody else lived there.
I swear on my life that this happened. The house is now renovated, and our favourite tree is cut down, but our memory of that day will forever be as if this happened yesterday.