WordPress Reserved Variables
Here you find reserved names which should not be used as variable names for POST, GET requests. Also it is not recommended to use them when creating custom post types, taxonomies etc.
Such names are created or specified when registering a taxonomy or new post type. Or when creating new rewrite rules.
Before you name a post type or taxonomy or other query variable, be sure to check if your name is NOT the same as the one of the list below. If this list has such a name, you need to come up with another one.
Below is a "black list" of query variables (forbidden variables in WordPress). Don't use the values below in query variables, because they are already used in WordPress.
All query variables
attachment attachment_id author author_name calendar cat category_name category__and category__in category__not_in comments_per_page comments_popup cpage day debug error exact feed hour link minute monthnum more name nav_menu nopaging offset order orderby p page paged pagename page_id pb perm post posts posts_per_archive_page posts_per_page post_format post_mime_type post_status post_type preview robots s search second sentence showposts static subpost subpost_id tag tag_id tag_slug__and tag_slug__in tag__and tag__in tag__not_in taxonomy tb term type w withcomments withoutcomments year
Public query variables
attachment attachment_id author author_name cat calendar category_name comments_popup cpage day error exact feed hour m minute monthnum more name order orderby p page_id page paged pagename pb post_type posts preview robots s search second sentence static subpost subpost_id taxonomy tag tb tag_id term tb w withcomments withoutcomments year
Internal query variables
category__in category__not_in category__and comments_per_page offset perm post__in post__not_in post_mime_type post_parent__in tag__and tag__in tag__not_in tag_id tag_slug__and tag_slug__in meta_key meta_value