I watched the live stream commentated by Myungwan Kim so I know what the parent is referring to (later in the game, Myungwan Kim referred back to that possibly being a mistake), although not being a Go player myself most of it was over my head.

It was fairly early in the game and about 4-5 lines down from the top and towards the center, center-left. Apparently, Lee Sedol played a little conservatively and "didn't take a ko" (?) when he could have.

Hope that helps you figure out what they're referring to, or maybe someone else can chime in, but that's what I remember.