Years ago now and shows how long I can hold a grudge so maybe doesn't reflect well on me but...
At university I was in a small knit group of friends. I got into an intense relationship with one of the guys, who swiftly broke my heart when I walked in on him in bed with one of the other girls in the group.
His story was that she was a dear platonic friend and they were just snuggling under the duvet together to watch a movie. I knew that I had definitely seen two people getting it on. He was my first love and I was gutted.
NO ONE in that group believed me. He was "such a nice guy...you're just so insecure, they are just friends...he loves you..." it was absolutely MADDENING. Even the girl who had been in his bed tried to tell me I had got it all wrong and just needed to trust him and not be jealous of their special friendship.
Obviously we broke up and he made a big deal of forgiving me for spreading such nasty rumours about him (!) and the narrative became that we had sadly broken up because of my insecurities!
Not long afterwards, another female friend from that same friendship group started seeing him. Being an intense bunch of nineteen year olds, she invited me for a drink to discuss it with me and to check it wouldn't affect our friendship.
I said something like "obviously I think you can do better, he didn't treat me very well."
She replied with something like (sorry this was years ago!) "oh not all that again, he said you would bring all that up, what happened is..." and pedalled out the same ridiculous bullshit he had given me about how I had not seen what I had seen with my own two eyes.
I was absolutely LIVID that he was still lying about it and making me look insane!
Anyway they got together, seemed very happy, and I was civil to him at group events, but never forgot what a dick he was.
A few years later, just after we all graduated, my friend found out that he had been shagging "platonic bestie" for the entire time they had been together.
I said absolutely nothing remotely I-told-you-so-ish to friend, and was genuinely sorry as my friend was devastated. But I couldn't help feeling vindicated as everyone in that friend group finally realised that I wasn't a madwoman!
Ex and platonic bestie are now married, everyone is fine and it's mostly consigned to water under the bridge uni drama, but I still think he is a massive gaslighting bellend, and was secretly delighted to see he had lost most of his hair last time we crossed paths a few years ago.