I spelled “derring-do” as “daring-do” in a recent post (I just went back and corrected it), only to read in a review of a modern sequel to Treasure Island about “feats of derring-do”. Oops!
So I googled it and discovered to my relief that only by a strange etymological twist has it come to be spelled “derring” instead of the original “daring”. “Daring-do” is quite common and may wind up the standard spelling one day.