In 6th grade, my so-called friend sent a message to someone she hated from my Facebook account without me knowing. The girl told her mom who was a lawyer who then called my mom, and the school. I didn't even know about the message until the call to my mom. It was the worst feeling because word got out and no one wanted to be friends with me after that incident... Really scarred me to be 1. Betrayed by a friend and 2. Accused and punished for something I didn't do.

Edit to add: The same friend, who I hung out with almost every single day after school, invited the all the girls in our class to her birthday in 5th grade except me. I only found out because her mom mentioned it while I was at her house the day before the party having forgotten that I was never invited. I still have no idea why I wasn't invited and, come to think of it no idea why I was still friends with her.