Earlier quoted context omitted.
That there are situations where women have affairs and get pregnant in very narrow timelines then have their partner think it’s their child. Putting the morality of this aside and speaking practically, unsuspecting men should have a DNA test if the timeline of conception feels off or extraordinary.
I would say it is more likely that consciously or unconsciously the woman started missing pills the month before. The couple was already considering it so it wasn't a big deal if she missed some pills. My wife and I were very fertile, we got pregnant on the first try every time we tried. I suspect that the birth control was barely working.
We weren't making it easy for the contraceptive either. I mean, several times a day, every day, for years... One of them was bound to get through eventually anyway.
I've understood that this kind of contraceptive makes only conception extremely difficult, not impossible. She was still ovulated (tested), and had regular periods.
It was extremely easy for us to conceive our second child. Now we have two pretty kids!
But we have to be really, really, really careful, lest we have yet another...