Alright... I've only told one person this and it was my girlfriend last night. Here we go.
When I was in eighth grade, I was in a group of friends with three guys and three girls. Once of the people in the group, let's call him John, was my best friend at the time and him and I were extremely close. Towards the end of eighth grade, another kid, Bob, joined our group of friends. Bob wasn't the best kind of guy (smoked pot, always had knives on him, etc) but everyone in the group really liked him so we didn't really think about it.
After a couple of months of Bob being in the group, I noticed that John and him got really close. I got jealous of Bob, and told John's mom about how Bob smoked pot, dealt drugs, and everything else about him KNOWING that she wouldn't let John hang out with him anymore. Well it went further than I wanted it to, and after about a week NO ONE was allowed to hang out with Bob. No one in the group knew that I told John's mom, and I felt terrible, but forgot about it pretty quickly.
Until freshman year.
It turns out I had lunch with Bob, and he sat at my lunch table. I remembered everything that happened with him in eighth grade, and tried to become friends with him to make up for everything before. We became friends, and he started to lay off the drugs, he got a girlfriend, and things were going great. Then one night he got drunk and sexted a random girl on kik who sent his girlfriend screenshots of all of their conversation. She broke up with Bob, and he got suuuuuper depressed. He would text me while be was high, and eventually ended up telling me that after we all stopped hanging out with him he tried to kill himself. Twice. He thought it was all John's fault and hated him for a really long time.
He still doesn't know it was me, and the only person who does is my girlfriend. I don't think I'll ever tell him.