Sensors, control systems, software, hardware and of course the weather. So many things can wrong despite your best efforts. Just think about it, the entire rocket goes through multiple cycles of extreme temperature/pressure variations in a very short span, the different materials used in the vehicle expand and contract at different magnitudes. You have got to get your calculations absolutely right. It is mind boggling to even think about it. Kudos to all the Engineers, Scientists and the support staff for pulling off such a feat. I feel lucky to be able to witness history in the making.