What is a fatal flaw?
A fatal flaw happens when two people have the correct polarity for compatibility of their sun signs (birthdays) but for other reasons are “compatibility challenged” due to a conflict from other factors in their Astrology charts. 
Common fatal flaws are as follows:
If A's sun sign is the same as B's moon sign; that can be responsible for a fatal flaw.
If A's sun sign is the same as B's rising sign; that can be responsible for a fatal flaw. 
If A's moon sign is the same as B's moon sign; that can be responsible for a fatal flaw.
If A's moon sign is the same as B's rising sign; that can be responsible for a fatal flaw.