While walking along a pond in the Pfrunger Ried (a mire in southern Germany), I came across a grove of birches. They're one of my favorite trees and I always spend time trying to place them in an appealing image. For this shot, I used my (at that time) brand new Canon 400/5.6 wide open. A great lens!
But I noticed a small reddish spot in this shot, along the edge of the right-hand birch, about a quarter of the way up from the grass. Can't figure out, what it may be. There was no direct sunlight, into which I was shooting. And it surely isn't colored dust on the sensor - though I had a hard time keeping the (original) 5D sensor clean. That's one reason I finally bought the Mk III version. Automatic sensor cleaning!