Mahesh's Eighteenth Law - Expertise

Expertise in one field does not imply expertise in all fields

Corollary
Expertise in one field does not imply expertise in any other field

Miscellaneous
Also called Condoleeza Rice Syndrome (She may may have been a tremendous success as the Provost of Stanford, but made for a lousy National Security Advisor)

And yet, we make this mistake all the time.  Marital advice from the police,  educational curricula from elected boards, Basketball players playing baseball, tax advice from Venture Capitalists, the list goes on...

Addendum
@caseyrosenthal points out that expertise may, however, be a necessary - but not sufficient! - condition...