fatal flaw

Especially if someone comes along and points out a fatal flaw.”

His fatal flaw, in fact, was his charisma.

It succeeds where the previous version failed, for a well-intentioned but nonetheless fatal flaw.

And after the debacles of the 2000 and 2016 elections in which Republicans won the presidency with victories in the electoral college while losing the popular vote, it's not necessary to make any argument that our presidential elections have a very serious, potentially fatal flaw for a modern democracy.

And yet Fox News has perpetually had competitors, and the fatal flaw with these competitors has always been that they are not Fox News.