There was a post about this perhaps six months ago. But in meantime, I had forgotten it, so repost it, since others may also have forgotten it:
HAMARTIA: Frequently translated as "tragic flaw," hamartia is simply a mishap or human frailty which leads to someone's reversal of fortune.

So, hamartia changes fortune for the worse. Is there an antonym? Not a 'fatal flaw', but a 'saving grace'?

I think it's the ordinary word for "fault", or failure, or flaw. I believe (don't know how to check) it's also the word for "sin" in the Greek New Testament.

It's also root of a medical word describing some astounding embryologic anomalies, in which recognizable malformed structures are found where they don't belong. I'll go search for information about this. The term is "hamartoma".

Search for "Pathology hamartoma" yielded this site:
http://wwwpath.usuf2.usuhs.mil/Surg_Path/s96-05/96-5.html

A bit more:
The word "sin" in New Testament Greek is "hamartia" = to miss the mark (as in archery), literally, anyway. So, maybe "aim true" or "sure shot" would work.