Both of my first daughters were surprises that occurred the first month I was careless (5 years apart).
This one I'm currently carrying took exactly a year of TTC. I actually wasn't truly expecting to become pregnant this month. After so long with nothing (other than a CP last summer), hitting the one year mark made me feel like all this effort was pointless. We did have sex once during my fertile period, but it was the morning after I'd gotten my positive OPK and I didn't have a strong feeling that we'd be successful. But then two weeks later I was a bit surprised to find that we'd finally done it!
I'm only 6 weeks pregnant now, and don't feel entirely confident that this will work out... But this is the most pregnant I've been in 8 years, so I'm trying to enjoy each moment.