At a party once where someone was taking snapshots with a digital camera. At one point, upon reviewing the pics taken to see how they turned out, and thinking of other ideas for pics, we discovered a weird phenomenon, a ghostly presence floating around some of the people in the images. It was a real puzzler for a minute or two, until we realized it was smoke from the cigarettes some were smoking. We had been in the room a while (the party lasted most of the night), so the smoke was not streamers from the cigarettes, but partially dissipated background. But nobody in the room noticed the smoke visually - we tuned it out.