Feet of clay

The phrase derives from the interpretation of the dream of Nebuchadnezzar, King of Babylon, by the prophet Daniel as recounted in the Book of Daniel:[2]

Your Majesty looked, and there before you stood a large statue—an enormous, dazzling statue, awesome in appearance.
The head of the statue was made of pure gold, its chest and arms of silver, its belly and thighs of bronze, its legs of iron, its feet partly of iron and partly of baked clay. (Daniel 2:31-33)
...
Just as you saw that the feet and toes were partly of baked clay and partly of iron, so this will be a divided kingdom; yet it will have some of the strength of iron in it, even as you saw iron mixed with clay.
As the toes were partly iron and partly clay, so this kingdom will be partly strong and partly brittle. And just as you saw the iron mixed with baked clay, so the people will be a mixture and will not remain united, any more than iron mixes with clay.(Daniel 2:41-43)

A well known instance of this phrase is in Byron's poem, "Ode to Napoleon Buonaparte":

Thanks for that lesson—it will teach
  To after-warriors more
Than high Philosophy can preach,
  And vainly preach’d before.
That spell upon the minds of men
Breaks never to unite again,
  That led them to adore
Those Pagod things of sabre sway,
With fronts of brass, and feet of clay.[3]

• Achilles' heel
• Hubris
• Ozymandias