The eligibility (Birther) issue has been raging since before Obama was elected. Bachmann needn't be "up on the issue" to know that millions of Americans have been openly wondering about the precise details of his birth and citizenship during the whole of his presidency.
As a potential political opponent to a sitting president, she ought to already have an above average knowledge of all of his weaknesses. The fact that she knows so little about this enormous defect on his part, speaks volumes about her unpreparedness for the huge task she's embarked upon.
That aside, simple political instinct should have prevented her from giving a single concession to Obama, no matter how small. She could have sidestepped Stephanopoulis' booby trap in any number of smooth ways, but she allowed herself to be tricked into easy submission.
That's not presidential. Neither is being a sitting Congresswoman, and not realizing that a president who's father was a foreign national, presents at least a potential conflict with Article II, Section I of the Constitution.
I say again -- Bachmann flunked the constant alertness test in that interview.