Using Twitter and Facebook, a man orchestrated a friendly snowball fight in Washington DC. During the fun, some people driving buy would provoke some of the participants and receive what they were asking for: a snowball. Some targeted gas-guzzlers. A certain hummer stopped in traffic and was hit with a snowball. A man then got out, brandishing a gun. It turned out that this man was a detective for the local police force.
He approached the crowd and began yelling at them to leave or he'll have them all arrested. Another cop arrived on scene, this one uniformed and on-duty. He also had his gun out, but only because someone reported an armed man in the area. He holstered his weapon when he found out the man in question was also a cop. The detective was then hit with a snowball and people began chanting, "Don't bring a gun to a snowball fight!"
After being hit with the snowball, the detective charged at the one responsible. He grabbed him and brought him over to his Hummer, where he slammed the man against the hood. The man was let go and soon after, police forced everyone to leave. The detective has been forced into desk duty until the incident has been investigated.
Source:
http://www.washingtonpost.com/wp-dyn/content/article/2009/12/20/AR2009122000881.html