A chick that hung around my group of friends for years, was boning a friend of mine, all the while, he had a girl pregnant that he was with, that none of us knew about. She knew about her, though. Dude's girl had the kid, he bailed out like a d**k. Girl he was cheating with (let's call her Nuts and Bolts. That's her nickname, due to her absolutely atrocious grill of a mouth.. looked like she chewed on nuts and bolts) leaves him for some unforeseen reason. Starts trying to holler at me. I tell her to f*** off. Starts hollering at my best friend. He tells her she's a slut and a homewrecker. I LOL. She casually hangs around us. Her best friend ends up dating the dude with the kid, not knowing any of his past (including his fling with Nuts and Bolts). They date for 6 months, she lives with him, figures out that he's a douche, and a dead-beat dad. She leaves him, and Nuts and Bolts hops back on that band-wagon, and is now dating dude again.
Also, for something that happened to me directly: was "seeing" this really cute (but far too young... she was 19) blonde girl. We weren't necessarily dating, but we were seeing each other, she was hanging out all the time, I sent her flowers, did the sweet nice guy thing. All seemed well, and I actually dug her a lot, age differences aside.
My band members and I (3/5 of us) were living in a house at this time. One day, out of the blue, the singer (notorious douche-nozzle. I hadn't learned this yet, though) comes up to me and says "hey.. just thought I'd tell you, that your chick and I have been talking. We like each other a lot, and we want to have a relationship. Sorry dude. Hope it doesn't effect our friendship/band". Needless to say, a few weeks later (and for other reasons) I beat the s*** out of him in the rocks in front of our house, in front of 40+ people at a party we were having lol.
Moral: People will do what they do. It's up to you to pick and choose how you will react to it. Now, none of this stuff would be worth my time.