A frantic call came in last week from a client running a high-traffic e-commerce store on a classic theme. They had updated to 6.9 because the dashboard offered it and they figured, “Why not?” Ten minutes later their product grids looked like a site from 1995: no styling at all, just raw HTML stacking down the page. Order confirmation emails had stopped going out too. That is the mess the WordPress 6.9 hotfixes are cleaning up.
My first thought was caching. I spent far too long purging Cloudflare and poking at their minification settings, convinced it was a concatenation glitch. It was not. The real cause was a change in how WordPress handles CSS for block libraries on classic themes. Assuming it is “just the cache” is the fastest way to lose an hour of your life.
The CSS mess behind the WordPress 6.9 hotfixes
WordPress 6.9 started loading block styles on demand to improve performance. On classic themes that meant the standard wp-block-library styles were suddenly omitted, so any content relying on them rendered unstyled. The regression is tracked in ticket #64354. A permanent fix is due in January, and until then you either run a temporary plugin or enqueue the styles manually. Performance work breaking backward compatibility is not a new story around here.
Then there is the email side. Core set out to make email more reliable, and as the official update at https://make.wordpress.org/core/2025/12/12/wordpress-6-9-hotfixes/ explains, the changes to the underlying mail libraries broke certain server configurations. If your site stopped sending mail after the update, you are probably hitting the PHPMailer bug. The core team updated the “Hotfix” plugin to cover it until the next maintenance release.
Fixing the adjacent post infinite loop
Adjacent post navigation changed too. Developers using the get_{$adjacent}_post_where filter started hitting infinite loops that crashed the server, caused by an unpublicized change in how the query is structured. If you run custom filters on post navigation, be careful about what you return as your WHERE clause now.
/**
* A safer way to handle adjacent post navigation filters
* to avoid the 6.9 infinite loop issues.
*/
function bbioon_fix_adjacent_loop( $where, $in_same_term, $excluded_terms, $taxonomy, $post ) {
global $wpdb;
// Check if we are in the middle of a process that could loop
if ( did_action( 'bbioon_processing_adjacent' ) ) {
return $where;
}
do_action( 'bbioon_processing_adjacent' );
// Your logic here...
return $where;
}
add_filter( 'get_previous_post_where', 'bbioon_fix_adjacent_loop', 10, 5 );Updating without the panic
Keep updating, but not straight onto a production site with no staging environment behind it. WordPress is a big ecosystem, and even a small performance tweak ripples further than you expect. If you are stuck today, the “Hotfix” plugin and the “Load Combined Core Block Assets” plugin will hold things together as a bridge until 6.9.1 lands.
This gets complicated quickly. If you are tired of debugging someone else’s mess and want your site to work without the Friday afternoon panic, drop my team a line. We have probably seen it before.
If your classic theme is still rendering unstyled blocks after the hotfixes, enqueueing the block library styles by hand is the next thing to try.
