Take a look at President Barack Obama approaching Marine One for his maiden flight on the helicopter, which he later described as a "spiffy ride" . The marine standing beside the door is at attention and smartly saluting his commander-in-chief . Mr Obama returns the salute - crisply - but then decides to shake the startled marine's hand.
I'm ex-Navy and did a fair bit of saluting and being saluted during my time and this struck me as plain wrong in terms of protocol. Over at Knowledge is Power, Colonel Jerry USMC (Ret) agrees: "Officers know that when approaching an enlisted Marine who is at attention, not to engage him in conversation or shake his hand without first saying, 'Stand at ease.'"
In the comments below the video, skipjak06 says: "You never saw a president handshaking his military guards in the past because it's not military protocol. That's because a Commander in Chief, returning the salute, or acknowledging it by a head nod, in itself is the proper and appropriate recognition of the Marine rendering that salute. The handshake was an amateur hour gesture. Did you see the look on that marine';s face? like, WTF???"
http://blogs.telegraph.co.uk/toby_harnden/blog/2009/02/06/did_barack_obama_breach_protocol_by_shaking_marines_hand