I had a turning point where my friendship with someone changed drastically.
When I first started my job in 06, I was single... so I was SO happy when I became friends with a girl I worked with who was also single, had a nice sarcastic sense of humor, and lived right around the corner from me. She very quickly became my "wing-man" and we would go out together all the time. We were like BFF in a matter of months, and since I don't have many female friends (and none that are single) it was so nice to have another single girl to go out with. We got along smashingly.
You could say that this girl is a bit of a drinker, and definitely a huge flirt in the most obvious way. Luckily for me, we liked very different sorts of guys. She would go for the guys who were what I would call "slick" and professional - wearing suits to work, going on dates in very expensive steakhouses, etc. Basically no signs of the geekery that I find so charming.
One night we were out at a local Tavern and she was drunk and flirting with a few guys. I had started talking to one guy who seemed JUST my type: smart, a little bookish, and a complete internet nerd. My friend had seen him when he walked in and literally turned her nose up at him. (she had a habit of sizing up every guy that passed her by)
So this guy and I are having a conversation for about a half hour, when my friend comes over and LITERALLY elbows me out of the way. She fucking shoved me. So she could get in his face and start flirting with him. I was completely astonished. My female friends have never been the man-eating type... they're pretty mellow and always put their girl friends first over guys. It was the only time in my life when I had the urge to punch someone in the face.
Instead, I sat on a bar stool and watched her do her thing. She was loud, crass, and in a matter of minutes (maybe 20) was making out with him in the middle of the bar. At one point she made eye contact with me in this completely evil "I won" sort of way.
From that point forward, I was still friendly with her at work, but we never went out for another night on the town. Fuck that shit.