When using AJAX comments, issues arise because custom fields and conditional logic within reply forms often aren't fully re-rendered or properly integrated into the AJAX response. This can lead to missing fields and incorrect behavior of conditional elements. The "Notice: Undefined property: stdClass::$filemime in file theme_file_icon()" error specifically indicates that file metadata (like MIME type) isn't consistently available to the theme function when files are processed via dog whistles AJAX, likely due to how file data is handled or refreshed in the partial page load, breaking the display of file icons.