You mean the horizontal stripes that look like video interlace lines?
Does the SX100 use an ultrasonic motor for its zoom mechanism? It might be some interference between its frequencies and something in the camera, or interfering with your memory card. Some brands of memory cards are more prone to interference from strong RF (radio frequency) signals. People shooting under or near noisy (RF noisy) sports-stadium illumination along with all their high-power RF communications systems find all sorts of artifacts in their digital images at times, later finding out that it is memory-card brand dependent.
No, wait, it's not your memory card now that I rethink this. If that were true, your ultrasonic motor, if you have one, would have to be operating while it is writing to the card. I bet its an interference between the zooming motor circuitry and your sensor's recording circuitry. If it is an ultrasonic motor, just its vibrations transmitting to the sensor might be enough to "shake" it enough during an exposure and cause odd results, from the ultra-fast physical motion alone.
Neat script! And sample photo! Thanks for sharing both.
p.s. Most good photo editors have de-interlace filters in their tweaking tools. Often used for people that are taking screen-grabs from video-input sources. It would clean those up pretty good.
