I have pcos, and before I fell Pg with dd1 I had only had 6 periods in my entire life (6 years after my first, so am average of one a year). We did use condoms, but to be fair, seeing as I knew I wasn't ovulating regularly, I wasn't as bothered by the odd condom mishap as I could have been. It took me 18 weeks and a small bump before I even noticed I was pg (my clothes stopped doing up). It was an incredibly unlikely pregnancy, as it was the first time I'd ovulated in about 18 months, and only one condom split that I knew of, but it happened somehow. Dd1 is now an adult.
Dd2 was planned, but we required help to get Pg that time. We were told it was very unlikely we would get pg naturally due to my my pcos (no periods), but not impossible (obviously as I'd already had dd1).
18 months after dd2 was born I fell pg naturally again. I'd only had one period since she was born, partly pcos, partly breastfeeding. That ended in miscarriage. I count that as an accidental pregnancy as we weren't trying at all, and we'd been told it was incredibly unlikely to ever happen. Funnily enough, since that pregnancy and miscarriage, I've had regular periods for 9 years, not used contraception at all, and have only had one pregnancy scare (turned out to be a medication issue rather than actually being pg). We've been very, very lax with contraception, happy to have another baby if it happens but not ttc. Nothing. Just luck, if I'd been using contraception I might have been congratulating myself for being so safe, but I haven't, so I'm obviously just not very fertile currently.