There are many. Here are a few that come to mind:
First: exact same story as you above, Groink. Almost comical. Except I did it to a fellow teammate on my college football team. It completely alienated me from my teammates for putting hoes before bros. I learned a very valuable lesson there that I never forgot.A buddy's girl (or fomer girl) should always be off-limits. Always. At least, that's the rule I learned and still live by.
Second: in junior year high school, I was playing in a band. The singer was a magnet for women. Like most singers, but he was also good looking. He could (and did) have any girl he wanted. He was dating this one girl for a long time, but cheating on her on the side with this girl I liked (who only viewed me as a friend). I was pissed about it. One day, the girl (the one he's cheating with) asks me if the singer was still dating his girlfriend. I could have said "no idea" or "no comment" or whatever. But I didn't. I told her the truth, hoping she'd like me instead for being a good guy. Bad move. She freaks, tells the singer's girlfriend. They both break up with him, and they both rat me out to the singer that I told them everything. I learned a lesson: never trust women in matters of the heart. And my first instance of bros before hoes. He never really forgave me, and I lost him as a good friend. And he really was a good friend and we told each other everything about our lives, but he could never trust me again.
Third: playing football in high school. I was a raging lunatic on the field, full of anger and hate. I hated everyone and everything including my own teammates. Well, I'm in my junior year, and my little brother makes the team as a freshman. Quarterback and everything, which I was really proud of him for. He eventually became the player I always wanted to be. He beat out the senior for the starting job, threw three TD passes in his first game (I was on O-line). Anyways, it's special teams practice before the second game of the season. Coach says run kick-off. I'm on kick off team, little brother is on kick-off return as a return guy. It was just supposed to be light, no contact, just run the lanes. I don't know what came over me, but I went full steam. Little brother catches the ball, I come sprinting down the field, and clock him. I was 250, he's 175. His finger dislocates (was gross), and he's out for 4 games. Doesn't start another game that year, and only regains his starting QB job in sophmore year. I will never forgive myself for doing that to my brother.
Fourth: dating my wife in college. We'd been going out about 5 months. It's halloween. I go as Superman (gay), and had just finished a bb'ing show the week before. Thought I was hot shit. She dresses up for the party as batgirl and we go as a couple. I ignore her all night, dance with a bunch of other girls. She says she's tired and wants to go home. I tell her "go ahead, I'm staying" and start dancing with some girl dressed as UPS driver (why do I remember this shit?). She just walks out totally humiliated. If it had been any other girl, I wouldn't care. But she became my wife. I can't believe I did that to her. I never did anything that night with any of the girls...just treated my future wife like a cast-away. Deplorable.
Fifth: bouncing at a bar. Working door. Some guy gets loud and drunk outside and calls me all sorts of stupid names. Usually I would just laugh and ignore him. This time I didn't. Just got up off my chair, and punched him in the forehead as hard as I could when I baited him to step up and get in my face. His face split open and he collapsed on the street (I found out later from the bar owner that he needed 23 stitches down the middle of his face to close the wound). Only reason the cops didn't nab me was they found weed on him, and figured that was enough for one night. That guy, drunk as he was, didn't deserve that. Drunk people aren't themselves, and I knew that. I don't know why I snapped, but I deserve my comeuppance for that.
Sixth: some of my posts on getbig. I don't know why I'm an asshole sometimes...in real life I'd never get worked up over being verbally challenged. There's no way I'd say the things I do in real life that I say on this site. This place brings it out of you though, doesn't it? So, sorry to anyone I've been a jerk to.