Will try to keep this short. Also not one of my closest friends, but someone I was good friends with for about 6 weeks before he did the biggest betrayal ever and the consequences lasted 2 years for me.
It's freshmen year of high school, first day. I attended a Catholic High School and 9th grade was the year you got a big mix of new kids from different middle schools and public schools. I get to school super early and see an unfamiliar kid also there very early, we're the only two there and we start talking and somewhat hit it off. We become friends and hang out quite a lot for the next month. However, I pretty quickly realize he is a total asshole and very arrogant too. Very manipulative, chronic liar, would throw people under the bus constantly, always bullying others, and full of himself to boot. I broke off the friendship after about 6 weeks.
Well... he starts spreading a rumor to EVERYONE that I was gay. He told the boys, the girls, even the teachers. His story was that we became good friends and I had confided in him that I was gay because I needed to get it off my chest and that I trusted him. The worst part was that people believed him because I was a VERY shy, awkward, and quiet teen, and people did know that we did in fact hang out quite often at the beginning of the school year.
Now before I continue, let me say I have nothing against homosexual people, but also understand this, at the time I was going to school, it was a bad stigma to be homosexual, ESPECIALLY in a catholic community at a catholic school. I knew kids who actually did come out to their parents and family that they were gay to literally be disowned and hated. So terrible these people were treated this way, and well, it was terrible for me too. My male friends stopped wanting to hang out with me because they thought I might be thinking about them sexually or they didn't want to hang out with a gay friend because of any possible association. Girls stayed clear of me too. I had teachers and counselors come talk to me asking me if i needed to talk to someone about my sexuality and even they didn't believe me when I said it was just a made up rumor and it had to stop. Sadly, everyone still believed the little asshole who kept telling the rumor. I confronted him about it and he just backed up his story and said to stop denying it and admit I'm gay. I would not.
The rumor that I was gay lasted my entire freshmen year of high school. My friends left me. Girls thought I was being crazy when I asked them on dates. I had to explain to teachers I actually was not gay. It took a solid year, almost two, (probably near the end of 10th grade) before the rumor was dispelled mostly to people realizing themselves I didn't really have any homosexual interests and that the kid who started the rumor was a lying bastard who also spread rumors about them. My friends slowly started to come back to me, and I started getting some interest from girls. But the damage was done. This one little rumor effectively ruined two years of high school for me... Even 11th and 12th grade were not as great as they could be because of small remnants of the rumor left with some students or teachers who were still not sure.
Like I said, I've been bullied a lot in my life, but it was always water off my back. This rumor however crushed me. It ruined two years of high school. It changed how people treated me and looked at me, and they all had it wrong. They say forgive and forget, but nearly 15 years later (I'm in my late twenties now), the bad memory is still as clear as day to me despite being married with kids. Lesson is: don't spread rumors!