Well I've got a few. How about mild first.

One girl I was off and on with for years wanted to get back together. I find out that she has a guy she was "kindof seeing" coming to spend the weekend with her on the night of... because she didn't want to tell him to make the few hours drive back home. I guess it didn't count as a boyfriend. I told her it wasn't okay. She said, "he is here", hung up the phone, and I didn't hear from her until Sunday night. I told her she was going to call and tell him right now, or I will send him our conversation history. It ended their semi-relationship. I think I did the right thing for him and I.

And for a fucked up one.

I dated a girl who destroyed my self esteem systematically. Took a long time to put myself together. Then just to rub it in, made sure I knew about the people she was fucking after we broke up. One night, we drunkenly hook up months later. Right in the middle, I lean into her ear and tell her it wasn't as tight as I remember. I remind her she has a boyfriend. All kinds of fucked up things. For some reason she kept going.

Yes, that was fucked up and immature.

That wasn't the last time I got her to cheat on boyfriends just so she would have to live with it. Looking back, it was absolutely manipulative and wrong. It doesn't matter what she did to me; I don't believe in eye for an eye with my own actions. I'll never feel good about it. It ate at me for years on a deep level. This was a friend from years before we dated. We are friends again, after a good bit of apologies for what we did to one another.

There are others, but those are the easiest offhand.