There have been a few about whom I've always wondered "What if?" as they approached me, but nearly all of them when I was already taken - although I would never have said yes in those circumstances and don't regret my decisions. There was one however who I wished the timing could have been a little better with.
I had just separated from my wife, but was still living under the same roof with her as I hadn't had time to save up to set up my own place yet. At that time, in preparation for an overseas posting with my job, I went to hospital for a CT scan. The operator was training a young Canadian student who to me was absolutely gorgeous, and I got the impression that she thought something similar about me, as we chatted up a storm and when I went to leave, she followed me out into the corridor where we kept talking. I sooooooo wanted to ask her out, but really, what could I do? When we're out on our first date and she asks me about where I live, what happens when I tell her that I'm separated from my wife but we still live in the same house? Oh yeah, I'm also about to disappear overseas for six months... I can just imagine how well all of THAT would have gone down!