The 'fatal flaw' is a literary term, related to the Greek word hamartia, 'error': the mistake a protagonist makes that leads to their downfall. The term derives from Aristotle's Poetics. The error that the character makes stems from a particular flaw in their personality (a 'fatal flaw'). This is usually an excess of arrogance which leads them to ignore the warnings or instructions of the gods. This particular flaw is called hubris ('wanton insolence').
An important feature of hamartia is that it is committed in ignorance. The flawed character is unaware of the impact their mistake will have, and ignores any warnings as to its effect.