I am using exiftool 6.31 without any serious problems. Recently I started using a development version of gimp which is built using version 0.6.13 of libexif. When gimp writes a jpeg file it appears to include all of the meta data. However, when I later use exiftool to update the User Comment field I always get the following message:

Exiftool fixes this problem when it writes the new comment, but I am wondering if someone can provide a more detailed explanation of exactly what problem this is indicating in the jpeg file so that I can help get the issue examined and resolved with libexif. Currently, the people looking at the libexif code see no problem. Thanks.
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Jeff