Not sure if this really relates to a secret:
When I was 12, we had a graduation for primary school, (grade 1 - 6), and my best friend (who was quite large at the time), wore the exact same dress as a girl in the popular group.
When my best friend wasn't looking I saw the popular girl (who wore the exact same dress) say to her friend "She looks so fat in her dress

"
At the time I thought it was a good idea to tell her. A huge mess happened because of that and both girls ended up crying. The popular b**** was in denial, and in the end no one believed me, including my best friend.
