This is a spot on the filter in front of the sensor in my Canon EOS 1D Mark II N. It could not be removed by blower, brush or sensor swab with Eclipse fluid. This spot was not on the filter when I sent this camera in to Canon for a shutter replacement. It causes a dark spot in all my photos in the lower left corner (lower right corner for vertical shots).

I shot this with my Canon EOS 1Ds Mark II with a 100mm macro lens on a VisibleDust Sensor Loupe.