Photobomb

pho·to·bomb - spoil a photograph of (a person or thing) by unexpectedly appearing in the camera's field of view as the picture is taken, typically as a prank or practical joke.

I was about to take a picture of my wife Cheryl, shopping at Trader Joe's, when this employee snuck up behind her and photobombed the picture.   Haha!  I think it turned out to be a cute picture of the two girls!