I submit for your consideration Euclid's fifth postulate. Given the amount of effort taken by people to prove it from his four other postulates, I consider it canny or lucky that Euclid chose to keep it as an axiom. Of course, it took unusual thinking and some discovery to realize that Euclid's fifth was indeed independent of the other postulates.
Gerhard "Ask Me About System Design" Paseman, 2010.06.10
