I saw this happen this afternoon, and it's always one bad apple, isn't it?
Chris Matthews was presiding over Hardball when Howard Fineman was making a point. With the camera on Fineman, he stopped mid-sentence, looked out behind Matthews and went "Whoa!"
They switched to the panel shot to catch a guy jumping Matthews, followed immediately by about four cops and several NBC folks. From what I saw, the guy didn't get much of Matthews, but they immediately switched to a long shot of the street.
Matthews took it like a champ, and with his mic still live, kept his composure and deftly handed it over to their reporter inside the Garden while they regrouped.
When they came back to the panel, the crowd in the background was replaced by cops on horses - and it's been that way since.