I fcked up big time in 2016/2017. My best friend (since age 7) really liked this hot blonde he met on the train, he was asking me how to text her, how to game her etc. but she obviously wasn't interested, they never went on a date or kissed.
I ended up meeting her in a pub with him (he randomly spotted her), I got her number and took her back to my place to bang that night. We banged met up for drinks/sex 2-3 more times until my friend revealed to her I had a GF (out of jealousy I presume). Anyway me and the blonde go separate ways, I stay with my toxic ex for a few more months - but always denied to my best friend I slept with that girl. In my mind, it was over so why hurt him?
Anyway, after finding out I had a gf, she proceed to pursue my best friend (after all this time friend-zoning him), they end up sleeping together, he gets besotted by her, thinks they are in a relationship (not the case) - anyway over the months I told a mutual about the blonde, who tells my best friend on his 30th birthday. He goes mad, screaming at me, literally frothing at the mouth, we don't speak for 4 months. He even tried to make a pass at my GF at the time, which I let slide as I didn't care about her anyway. but I think that is worse than what I did.
In his mind, I slept with his girlfriend, in reality, I only banged a girl he was attracted to... and subsequently had sex with himself. She even admitted to only banging with him out of spite towards me. My best friend still brings this situation up, in fact he threatened to bring it up last month in front of my GF as a way to dominate me, however, I'd told her the whole story already.
He needs professional help, over the couple of years that have past, he has convinced himself I did something terrible.
We made a pact never to go after the same woman, but he still can't let it go. He doesn't have the best luck with women so maybe I should have not gone there... but she wasn't interested in him at all.