I'm seeing a SQL error in my log file that reads like this:
[15-Oct-2016 17:34:59 UTC] WordPress database error You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WHERE AND comment_parent IN (10) ORDER BY comment_date_gmt ASC, comment_ID ASC' at line 1 for query SELECT wp_comments.comment_ID, wp_comments.comment_parent WHERE AND comment_parent IN (10) ORDER BY comment_date_gmt ASC, comment_ID ASC made by require('wp-blog-header.php'), require_once('wp-includes/template-loader.php'), include('/themes/smb/single.php'), comments_template, WP_Comment_Query->__construct, WP_Comment_Query->query, WP_Comment_Query->get_comments, WP_Comment_Query->fill_descendants
The error occurs on a post that does have child comments, and our theme does show those in nested fashion, but very generically, using the wp_list_comments function. The theme isn't using any special hooks, or any other tricks I can see.
Googling for this error, I see it show up on other people's blogs (where it is getting inserted into the output page! At least on my site I don't see that), so I know this isn't unique to our site.
So the question is: does anybody know what produces this error, and how to fix it?
We're on Wordpress 4.6.1 and the theme is custom built.