The popularity of social media has caused a spike in the art of photobombing, which is the act of throwing yourself into someone else's picture just as it's being taken. Still not sure what that is? Check your snapshots from your most recent vacation. See that stranger in the background making a face at the camera? He's photobombing you.

But even some of the world's biggest pop stars are not exempt from this, both as photobombers or photobombees. Take the case of R&B singer Usher, for example. On Dec. 30, he posted a composite of four photos of himself to the popular photo sharing site Instagram, where he goes by the screen name of "howuseeit." In the final picture, walking in front of him is the unmistakable face of Bruce Springsteen.

"Was in NYC and trying to get a cool photo of me in the reflection and this guy photobombed me," Usher wrote in the accompanying text. "I would be mad but it was THE BOSS! Photobombed by THE BOSS! #cools---happensinNYC"

More than 6,700 people registered their approval with the picture, with more than 230 comments. Although Usher thought it was funny, we noticed that he was not listed among those who will be performing at the 2013 MusiCares tribute to Springsteen.