By now I’m sure you know about the “Trinity scientists worrying whether the test would set the atmosphere on fire” story, although this post is the most complete examination of the story I’ve found.
One interesting tidbit: the fireball was much brighter and longer-lasting than James Conant had expected, and so for a moment he thought it had really happened:
Then came a burst of white light that seemed to fill the sky and seemed to last for seconds. I had expected a relatively quick and bright flash. The enormity of the light and its length quite stunned me. My instantaneous reaction was that something had gone wrong and that the thermal nuclear [sic] transformational of the atmosphere, once discussed as a possibility and jokingly referred to a few minutes earlier, had actually occurred.
Everyone has had those brief “oh shit!” moments of pure panic, although I’m willing to bet no one has ever had one quite like James Conant briefly thinking he had just annihilated all life on earth.