Members of the Canon S1 newsgroup on Yahoo have been working on a bug involving timelapse photography (called the "intervalometer" by Canon).
garymansell set up his S1 to take five photographs with flash (one minute apart). The camera appeared to take the five photographs, but only the first and last photos showed up when viewed on the camera.
Members of the newsgroup made a variety of suggestions. Was it the problem related to firmware revision? No. Were the batteries too weak to handle five flashes? No.
technicsplayer was able to reproduce the problem using a 2GB memory card; the problem did not occur when he used a 512MB card. He wonders, "So it must be a (large?) card compatability problem and canon needs a firmware fix for this? Both cards had been formatted in the S1."
The five photos are recorded by the S1is, and show up when the memory card is removed and placed in a card reader. The bug is that the camera fails to show the three inbetween pictures.