A very basic one is scripture – the Bible teaches both free will and predestination/election/chosenness/etc. As has already been made abundantly clear and accepted by all comers, including myself, they are incompatible… in linear sequential time. (I say ‘linear’ because we know from physics and relativity that time can be bent, compressed or warped – the twin paradox is a good example. It should not be too surprising nor difficult then to understand that our sequential time is not an absolute.)
The other major reason is my life-experience, I guess. The first ‘co-instants’ that knocked me over is the one below. There was another whole set of co-instances that I realized in retrospect that was far more significant to my life’s path – my chasing my future wife around the country, unbeknownst to either of us, at the time, and finally catching up with her (she reminds me that I still am sometimes 
). Part of that set had already occurred at the time of this story: