I have, on several occasions, and this girl had a boyfriend when I met her. I didn't pursue this gf at all for the first 6 months I knew her because any of it, but the more distant her and her bf grew (he lived 3 hours away) and the more time I spent with her, the more I put myself and her in a position (her doing as well) to get to the point where we could be more than friends. I'm pretty sure I could've slept with her earlier than I did (especially now that I know what she's capable of), but I refused to pursue her physically until she was out of the relationship. Of course because of his distance she was basically casually dating me for a month before he even got dumped, and slept with me the day before she broke up with him. She stayed over a night, I wouldn't do anything, the next day she asked to stay over again and convinced me as soon as he was back in town (the next day) it was over. Maybe I should've waited, but I "plowed" her that night. Obviously this on top of her mom being remarried 6 times shows a super destructive pattern in regards to relationship, and I ended up 'getting mine' in the end as well. I bet the dude I 'took' her from is tap dancing his ass to work every morning.
The usual 'call for a good time' list has definitely been hit up in the last week, and the ones who are with people I am not pursuing for anything at all. Not that I ever wanted to make a girl on that list a girlfriend ever, but karma is starting to feel a little real lately.