The ISOCAM flux calibration is derived from stabilized images of a set of ISO calibration standards which were observed towards the center of the array where flat-fielding affects are negligible. Thus, if your data has not stabilized, or flat-fielding is an issue, the flux calibration is likely to yield inaccurate values. The degree of inaccuracy depends upon the degree of stabilization (see TRANSIENTS) above. The flat-field induced errors are a significant contributor only for targets which are near one of the corners of the array and which are observed with the 6.0 and the 12.0 arc-second pixel-field-of-view lens and for which no flat could be derived from the data. If you have reason to believe that the data have not yet stabilized one possible solution is to use the location of the so-called ``fast-transient'' (also known as the ``initial jump'') to calibrate the fluxes. This method is less reliable and more information is available by contacting the ISOCAM support scientist.
It is also a good idea to calibrate the behavior of complex reduction algorithms by inserting fake-sources (excellent ISOCAM point-spread-functions, PSFs, are available for this purpose).