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don't think anybody knows.. lol
i found this by searching for invalid default value for 'cat_id'
search resultif you want anymore help i suggest you actually give more information for those of us here who aren't Psychic.
things like XOOPS version, php version, MySQL version, and last of all which would probably really help you a lot is to mention where you are getting that error message and with what module because there are hundreds of modules that use cat_id in their MySQL table structure. we can't help you until you tell us as much as possible.