Failure of the Calendar Method

I've had regular, monthly, fairly predictable menstruation cycles for what seemed like forever, and I'll admit that for a long time, I've relied a lot on the calendar method for contraception. This method, also called the rhythm method, assumes that you will be able to predict your "fertile days" based on the average length of your cycles. So, if I could have predicted when I was due to ovulate, then why did I get pregnant and why am I now suddenly a mother? What went wrong, or you might say, right?