My single favorite photo from 2012 was an accident. While in Istanbul, Turkey on vacation, I woke up early on June 6th to try to photograph the Transit of Venus - the last for more than a century. From this part of the world, the transit would be visible only at sunrise.
I had a thick filter (a piece of welding glass) that I used for several photos of the transit. Some of those photos came out OK. My best photo, however, was this one that I took for fun, without a filter, just as the sun began to emerge above the clouds. I wasn't expecting this photo to show the shadow of Venus, but - to my surprise - after reducing highlights and increasing contrast, it produced a remarkable image.
(Another fun fact about this photo was that I was standing in Europe, but watching the Sun rise over Asia!)
(June 6, 2012. Nikon D5000, 1/1600s @ f/9, 300mm, ISO 200.)