There is a certain amount of play in ISOCAM's wheels and the image of the Fabry mirror can occasionally shift the field of view off of column 0 or 31 when using the 6 arcsec pfov. The effect is relatively uncommon, and column 0 is more often affected than column 31. The blockage is only seen in LW, but may be present in SW as well where the background is too low to clearly detect the edge of the field of view.