When you run the output from list.e (i.e. NEW_IMAGES/download.b) you get an <Name>.IMG file for each image, even if there was an error in the download. If you don't notice there was an error and run convert.e, you get an error that looks like this ...
<rand> Iapetus$ ../CASSINI/UTILITIES/convert.e INPUT LIST IAPETUS.TXT At line 118 of file convert.f (unit = 10, file = 'NEW_IMAGES/N1483150865_1.IMG') Fortran runtime error: Non-existing record number
This happened to JRW because the directory structure on the PDS server is different from that in cumindex.tab. You'll have to re-download the files.