Not a “gotcha” question but definitely a “gotcha” moment. That was an un-micked print interview recorded from a still video camera. I'm sure Cain had no idea that it would be released for public consumption and as such let his guard down. In the end his answer wasn't bad, the problem was getting there.
Even taking into account that his guard was down, there is no excuse for Cain looking as lost as he did. The ‘I disagree for the following reasons . . . wait, no, that’s a different line’ moment is what did it for me - it came across not as being thoughtful and deliberate, but as trying to recall the right canned answer.
And there was no reason for Cain to let his guard down - he was doing an interview with an MSM editorial board. If the camera wasn’t there, the editors still would have reported that he had trouble coming up with a coherent response to the Libya situation; the only difference is that, now, they have video evidence.