Achilles heel
From Tardis Wiki, the free Doctor Who reference
An Achilles heel was a weakness, typically of something otherwise strong. The Eleventh Doctor became annoyed at the lack of such a flaw in the Shakri cubes. (TV: The Power of Three)
A Nimon once mistook the Eleventh Doctor manipulation for revealing his Achilles heel. (COMIC: Space in Dimension Relative and Time)
