As an expression or idiom, feet of clay implies an underlying weakness or fault, or a character flaw that is not readily apparent : “They discovered to their vast discomfiture that their idol had feet of clay, after placing him upon a pedestal” (James Joyce, Ulysses) 1
And from Theoretikos, by Oscar Wilde:
THIS mighty empire hath but feet of clay:
Of all its ancient chivalry and might
Our little island is forsaken quite: 2
The origin of the feet of clay metaphor* is from Nebuchadnezzar's dream in The Book of Daniel, in reference to the statue of an idol : “His legs of iron, his feet part of iron and part of clay.” 3
The ever popular Dictionary of Cultural Literacy adds:
People are said to have feet of clay if they are revealed to have a weakness or flaw that most people were unaware of: “When the coach was arrested for drunken driving, the students realized that their hero had feet of clay.” 4
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1http://www.quotesandsayings.com/finquoteframes.htm ; see also http://www.freebooks.biz/Classics/Joyce/Ulysses/Ulysses_16_13.htm
2http://www.bartleby.com/143/9.html
3 John Bartlett, Familiar Quotations, Little, Brown, and Company, 1980, 32:32
4 E.D. Hirsch, Jr., et al; The Dictionary of Cultural Literacy: What Every American Needs to Know, Houghton Mifflin Company, 1993, p. 67.
I remember hearing Hawkeye Pierce using this expression on M*A*S*H when I was a kid, and being confused by it. Node what you don't know.
*Thanks to Tiefling for helping clear up the Bible reference.