You are misinformed--she's never been at ANY Yankee game, ever. You bet she was lying, but it goes beyond that. She'll never set foot in the stadium because there has always been the possibility she'd be booed if her image was shown on the screen to the crowd.
This was a major, major slip-up. Perhaps Hillary thought the rock & roll crowd was safe. But normally she just doesn't go ANYPLACE where the crowd's reactions can't be controlled or edited out later.
I said it on another thread--live TV is not Hillary's friend.