Ground Zero - S4-E14
Other mistake: When Hawke blows up Yahara's plane, we see that the video feed in the holding cell where Locke, Santini, and Rivers are is actually from a 3rd person perspective and that it continues, but according to previous dialogue (and video up to that point) the camera was installed on the plane, so once Hawke blew the plane up, there should have been no more video feed. (00:40:35)
Answer: I don't think it's ever explained why Mac refused to tell Hawke where St. John is.