Something i came across online,,,
Quite funny.
"Upon reviewing the flats, one stood out, with a very weird artifact in the image. It took a second, but the artifact's cause then dawned on me. I had been riveted by the upper LCD display on the camera during the image acquisitions, waiting for the shutter speed display to vanish before I pressed the shutter button for the next exposure. Any day now, one of the second batches of Timelapse+ View intervalometers (http://www.timelapseplus.com/view/) will show up, but for now, my finger was triggering the exposures. Bottom line, I was pretty oblivious to transient sky conditions during the flat sequences. During a swim in the pool afterwards, I noticed a number of bats wheeling over our backyard. Well, that explains the artifact, all right! The exposure was 1/15th of a second, and the bat had a definite vector velocity and high-speed wing-flapping that generated the dark line and lighter wing zones. "
Roger