There is a list of words or terms in WordPress which should not be used under some circumstances as they may cause conflicts with WordPress Core functionality. Using such reserved words or terms may result in some odd errors such as unexplained 404 redirects, template reverting to index.php when it should not or form data not getting saved.
If such reserved words are used, it may cause some issues that are very hard to debug as WordPress would not provide debugging information that will be useful. For example, when adding a reserved word to the query arguments, it may not resolve as required and would lead to a 404 error which will be hard to debug especially when added using an interface that works with other words or a code snippet that is logically correct. In this case, pointing out the issue will be very difficult because there will be no obvious issue in the logic of the code unless one knows the issue related to reserved words, it will be hard finding what the actual issue is.
Avoid these words in the following conditions:
- Passing the word to a $_POST or $_GET request array.
- Registering a post type or taxonomy
- Handling query variables
This applies both when doing this in code using a plugin that allows you to this in a user interface, as long as the underlying code will conflict with WordPress core, then those words should not be used.
Should you encounter some unknown issues when using such words, the easiest solution is to prefix such words, like ‘book_author’ instead of ‘author’, but the best solution is to avoid using them altogether or use their synonyms if preferable.
Reserved words in WordPress core
attachment attachment_id author author_name calendar cat category category__and category__in category__not_in category_name comments_per_page comments_popup custom customize_messenger_channel customized cpage day debug embed error exact feed hour link_category m minute monthnum more name nav_menu nonce nopaging offset order orderby p page page_id paged pagename pb perm post post__in post__not_in post_format post_mime_type post_status post_tag post_type posts posts_per_archive_page posts_per_page preview robots s search second sentence showposts static subpost subpost_id tag tag__and tag__in tag__not_in tag_id tag_slug__and tag_slug__in taxonomy tb term terms theme title type w withcomments withoutcomments year
This is not a full list of the words or terms, visit the Reserved Terms page in WordPress Codex for a full list of reserved words or terms.