Lay unnoticed until now, that is. Only when it's too late to correct does it leap out of the page. As the number of words in a manuscript tends to infinity, the probability of a significant error making it into the final version approaches 1. I've written 23 books and several dozen long research papers in my career, and the same thing has happened every time. The Maths Gene arrived on my desk earlier this week.
Publishing is like that.
Don't ask why. But, experienced as I was, that first glaring slip still caught me off guard. In fact, you need just 23. If there is no restriction on which two people will share a birthday, it makes an enormous difference. Now the birthday problem is one of my favorite mathematical examples.