brit milah - Twin Bris conundrum

Twin brothers are born from the same mother on the very same morning. Both are perfectly healthy. Yet, the proper day for one’s bris mila is 8 days later, while the proper day for the other one’s bris mila is not until the 9th day. Why?

Answer

It was Shabbos morning and one was born naturally and one one was born via a c-section. The one born naturally has his Bris the following Shabbos, the one born via c-section has his Bris on Sunday the 9th day.

http://www.chabad.org/library/article_cdo/aid/634611/jewish/Can-a-circumcision-be-conducted-on-Shabbat.htm