In 1944, Ansel Adams patiently waited for a distant horse to pose in the perfect position for an iconic photograph of the Sierra Nevada range. In 2009, this photographer had different experience with two swans while capturing a formal portrait of North America's highest peak...
"Steady... Steady... DIVE!"
And thus the elegant swan pair at the far left conspired to photobomb the image by going ass-up at the exact moment the photographer released the shutter.
"Hmmmmmm..." thought the photographer upon later review of the image, "I suppose one shouldn't take these things too seriously anyway."