I think we grow up with this idea that any one slip will result in pregnancy. While that is true, the reality is that most woman are most fertile in a small window of days each month. So, if your "slip" doesn't happen in that window of time, you didn't really dodge a bullet, you just weren't likely to get her pregnant anyways. But it's irresponsible to preach that or let people know that's a likely outcome for unprotected sex. Better to tell them the story about the guy who got his girl pregnant while she was on her period.
I learned that when it was time to make some babies. I thought we'd immediately get pregnant just by getting off birth control and having sex like we normally did. Nope. After a few months of not getting the results, we actually mapped out the days on the calendar when she was most fertile, had sex every day in that period of time and then got pregnant.