Done quite a few things that I regret.
I purposely stole girlfriends from a friend. I've stolen 4 girlfriends from him. I acted like I didn't do it on purpose and that they came to me, but I did it on purpose because I thought it was funny. And after I got them, I would get incredibly bored and then ignore them. Himuro knows the guy, Justin Block. After the first 3, I was able to get him to forgive me. But when I stole his 4th girlfriend (the ugliest one too *shudder*), he got pissed and refused to speak with me.
Another incident, I had two friends that were dating. The girl and I were involved a few years prior. We eventually ended it, but the sexual tension and attraction was obviously still there. I got jealous after she started dating him. So I would make him look like an ass in front of her and the guy had no spine. He would never stand up and eventually she noticed this. I would even rub her legs and stuff while were all hanging out. I would constantly hit on her. She eventually decided that he was a vagina and wanted to break up with him. I went with her to his house to break up with him because I insisted I should be there. I just knew it would be funny and drive the point home. When we were leaving his house, I put my arm around her and said "Let's go back to my place, baby." Then he went around telling everyone he wanted to kick my ass for it. So one night he's at a party getting drunk with my friend, and he starts talking shit and says he's going to kick my ass. My friend, knowing this guy doesn't stand a chance, pushes him to go to my house and even gives him a ride. I open the front door and he starts talking shit. I broke his jaw and rolled him off my patio and went back to fucking his ex g/f.
The one I regret most though had to do with a girl I cared a lot about. She was my first love.

We dated for a long time until she had to move to Dallas with her family. She would call me all the time and tell me that she was really lonely and had no friends and that she missed it. I mistook it for her just getting used to a new place. I kept telling her it would get better and I pushed her problems in the back of my mind. One weekend she drove to Houston unexpectedly and called me to surprise me. I was really drunk and with some friends and told her I was busy that I would hang out with her the next day. She showed up at my house that night and started talking about how I never had time for her and stuff like that. Keep in mind, I really did love her and had her best interest at heart. I just didn't know how depressed she was. I got mad and yelled at her and told her that she should have called first. She left back to Dallas. A few days later I was getting drunk with a friend and I got a call from one of her friends in Dallas saying that she had committed suicide. If I had only taken the time to talk to her and understand how bad it was, she would still be alive probably. And I really did care about her. The other people I didn't give two shits about. That event fucked me up really bad and led to drug addictions and lots of things that I really really regret.
All of this shit was done during a time period when I was on lots of drugs and just on a horrible path in my life. I was going nowhere. I regret all of them. Good stuff. I'm a fucking asshole.