It's a prank favoured by kids and college students but today President Obama was an unwitting culprit. He committed the ultimate photographic faux pas while posing for pictures with other leaders during the Open Government Partnership event at the United Nations, accidentally blocking the face of the man to his right while waving. The act of hijacking somebody's photograph is known as 'photobombing' and the most common example is for a person who is not supposed to be in the picture to pull a silly face behind the people posing.