What is a fatal flaw?

A fatal flaw is a characteristic of a thing that thoroughly undermines it. If you say that an argument has a fatal flaw that means that either some essential fact is wrong or that some logical fallacy has been employed that undermines the validity of the conclusion. For instance if I say all cats are black, Felix is a cat, therefore Felix is black. 

 

The fact that it is easily demonstrated that some cats are not black undermines my conclusion. If I say all cats are mammals, Felix is a mammal, therefore Felix is a cat, I have drawn a conclusion it supported by the facts so my argument is fatally flawed.

 

In literature, a character can be said to have a fatal flaw, meaning a characteristic that leads to their downfall. For instance, Hamlet’s indecisiveness is the fatal flaw that eventually leads to his death.

A plan can also be fatally flawed in literature. With everything being undone by some oversight. A lot of mysteries work that way. With some elaborate crime eventually being unravelled because of a small mistake.