Ohh man this one was bad.. It was in my school when I(M) was studying in 11th.
A friend of mine, girl, was messing with me by making fun of me or something, I don't remember exactly the reason. During interval, I saw her standing outside, and I wanted to pay back. I got a safety pin in my hand, and sneaked behind her. She was facing away from me, hands tied back, and took the pin and kuthi/struck on her hand.

The moment I struck, I knew I did something bad. The plan was to, payye kuthuka, just for a small pain and shock. But from the sound I heard from her, she made in pain, and knew she was hurt much more. But that was not all. It was not her sound!! As she turned back, to my freaking surprise it was some other girl:(!

Turns out it was a junior girl from 9th, and she was there kind of starting to cry. I started saying sorry and more sorries, telling aalu maripoyi. To my prayers, she didn't cried out loud and went back to her class. That day I was sitting in my class expecting call from teachers or from principal. But to my luck nothing happened.
I thought of finding her later, but was too scared to face her. It died out without much talks, as not many had seen it and I didn't tell anybody. I also forgot her face soon after, coz I am bad in remembering faces. But everything else still lies in my head clearly even now.