Sorry for the wall of text. I promise the only monster at the end is the person who betrayed me.
In high school, I dated a really awesome guy who had a female best friend. As far as I could tell, she was really cool. I'm not the type to have trust issues, so it wasn't a problem. But since she was my boyfriend's best friend, I went out of my way to be super nice to her and befriend her. If she's important to someone I love, then she's important to me, you know?
We became really sort of close. I talked to her when I had trouble with my boyfriend or didn't understand something and she helped me understand him. And more so his friend group (who was very unlike him).
So my boyfriend called me one day and broke up with me, totally out of the blue. When I talked to my friends about it, they hesitantly told me that his best friend had told a bunch of people that I was jealous of her and my boyfriend. This was news to me, as she and I had always gotten along and it had honestly never crossed my mind that she was a threat.
I figure she was misinformed and I apologize for any misunderstandings that occurred between us. My boyfriend and I continue to be close friends, as we were before we dated. One night I'm joking with his best friend about how ridiculous he is. I called him an asshole (all very affectionate), then asked that she not repeat that to him in case it comes across wrong. She immediately texted it to him. He called me and told me that I really hurt him and he didn't know if he could forgive me for talking behind his back.
I didn't really talk to her much after that and I begin to see that she was kind of awful.
Shortly after, she began dating my ex. Obviously, it's a bit painful, but at the same time it's nice to see my friend so happy. I know my ex was a little hesitant about the possibility of it hurting me, and I figure that hesitation might trickle into his new girlfriend (especially since it had barely been a month since we broke up). Having a difficult time expressing myself verbally, I wrote her a letter to explain that I don't mind so long as she treats him well. I gave it to her. She read it, shrugged, and walked off.
About this time, his friend group starts to fess up to what led up to the breakup. They were previously neutral about the issue (fair enough) but they had become really uncomfortable with how the new girlfriend/previous best friend was treating the situation.
Not only had she told everyone that I was jealous, she had been feeding my ex and the friend group shit that I was trying to keep them from being friends, that I had told her to "stay away from my boyfriend."
Bad news: A lot of people lost respect for me because of her. People remembered throughout my high school career and occasionally brought it up. Not willing to badmouth someone in a public setting, I shrugged it off.
Good news: My boyfriend and I eventually made up and became good friends. He broke up with his girlfriend, came out as gay, and is now married. I haven't seen him in a few years, but I talk to him sometimes and he is doing very well. I came out as bisexual/gay as well, so ultimately our relationship wouldn't have worked. But that time in my life really sticks with me as being incredibly happy, even if it ended in rather public humiliation. He's an incredible guy and I'm lucky to have him as a friend.
TL;DR: Boyfriend's best friend manipulates people and causes me public humiliation.