Last night an attacker wearing a hood jumped the security line where Chris Matthews was broadcasting from and attempted to attack him. He was quickly removed by police.
Perhaps broadcasting from a location practically in the street just wasn't the great idea someone at MSNBC must have thought it would be.
You can see the video here: http://tiger.towson.edu/users/bstelt1/attack.mpeg
I've noticed that sometimes it doesn't work the first attempt, you only get about a second of the video, but a second attempt is usually successful.