There have been a few in my life...but a couple stand out.
Two acquaintances of mine in my early twenties come to mind. They were not a horrid dating match, but clearly not a life match, either. Maybe one of those situations where you date that person for a while, learn something about handling a real, grown-up relationship, but break up after a while. But they stayed together because of an accidental pregnancy. Their kid came out weird: Giant head, big ol' birthmark on half the face, and freakishly HUGE baby. I didn't hang out with them much after that because they had no time, and inviting them cramped what we could do and when. Last I saw, their giant baby grew into a freakishly huge toddler who didn't talk much and constantly had a wild, glazed look in its eyes. Something was off about that kid. And last I heard, the couple were pretty blah about each other...not antagonistic, but clearly bored and just being polite/doing the right thing for the kid. I could already tell that they were either going to split up, or end up cheating on each other. I couldn't imagine having my life all boxed up that way at only 22 years old.
My best friend's older sister got knocked up by mistake in her late twenties. She told her boyfriend about it, and he made it abundantly clear that he wanted nothing to do with fatherhood and she would be on her own. So what did she do? Had the baby and kept it, of course! Mental health issues revealed themselves when the baby was still an infant, so Best Friend's parents wound up raising it. Best Friend's sister had zero interest in trying to get back custody of her son. The kid had a lot of issues as he got older, and frankly Best Friend's mom wasn't up to raising another kid, let alone a troubled teen with learning problems. Harsh as it may be to say it, that kid should have been aborted or adopted out. Best Friend was already CF, and watching that drama made her doubly so. Best Friend's little sister and brother still lived at home when the grandson arrived, and it left both of them deeply cautious about kids. Best Friend's younger sister still doesn't have any kids at 35, and brother did wind up having two--but was very, VERY careful to make sure that he and his wife both wanted 'em and had the time and money to support them. Older Sister STILL is unable to function on her own...lives in gov't housing, can't keep a job, and gets annoyed when her son refuses to call her "mom"....so safe to say that she hasn't learned a damn thing. Whole thing stiffened my CF resolve.
I've also watched a friend who was very involved in the arts (acting, directing, vocal music, writing, nude model for life drawing classes) completely squash away that part of herself. Theatre was the first thing to go--no shock there, since it devours any spare time you have. Vocal music went at the same time, as she decided to forego any lessons or ensemble singing. She removed any art in their house that was not child-appropriate, so of course she didn't model anymore. She stopped writing, too, though she did manage to use National Novel Writing Month to scratch that itch a couple of times. She's talked about writing a book or collection of essays a couple of times, and I've offered to help her revise and edit her work...nothing forthcoming. I myself haven't been a globetrotter or anything, but it's clear that we've kind of switched lives in some ways. She chose to give up her favorite creative endeavors for a rather conservative suburban crunchy-yuppie life. I was more restrained and naive in college, but now I'm out having adventures that my friend at college age would have loved. She'd probably love them now, too, if she'd accept that it's okay for people to have adult-level interests after they have kids, and that those adult interests make you a better and happier parent. Oh well. I've invited her for visits and along on some of my outdoor adventures---even offered to plan a boat trip with/for her, around her schedule and of course all I got from her end was a wistful, regretful "no thanks". I can't imagine trading in such a large chunk of what makes me the person I am today.