I've never seen this before, but as I was looking for dust spots on my sensor(found a few), this appeared. I tried to duplicate it at several different apertures but couldn't find it again. It only showed up at f/16, so I took the camera out and tried f/16 again and several other apertures but it didn't appear again. Hopefully it's a one time occurrence, but if any of you have any advice, I'd appreciate it.
Screen Shot 2012-05-17 at 2.18.59 PM by Winginkris, on Flickr