I got a frantic call last week from a client running a high-traffic e-commerce store on a classic theme. They’d just updated to 6.9 because they saw the notification in the dashboard and figured, “Why not?” Ten minutes later, their product grids looked like a site from 1995—no styling, just raw HTML stacking. To make matters worse, customers weren’t getting order confirmation emails. Total nightmare. This is the reality behind the latest WordPress 6.9 hotfixes that many of us are dealing with right now.
My first thought? Caching. I spent way too much time purging Cloudflare and tinkering with their minification settings. I was convinced it was a concatenation glitch. But that wasn’t it. The issue was actually a fundamental change in how WordPress handles CSS for block libraries in classic themes. Trust me on this: assuming it’s “just the cache” is the fastest way to lose an hour of your life.
The CSS Mess and WordPress 6.9 Hotfixes
WordPress 6.9 introduced a feature to load block styles on demand to boost performance. It sounds great on paper, but for classic themes, it meant the standard wp-block-library styles were suddenly being omitted. This left content that relied on those styles completely unstyled. This specific regression is being tracked in ticket #64354, and while a permanent fix is coming in January, the current workaround involves either a temporary plugin or manual style enqueuing. It’s a classic case of performance optimization breaking backward compatibility.
Then there’s the email situation. Core tried to make email more reliable, but as I saw in the official update at [https://make.wordpress.org/core/2025/12/12/wordpress-6-9-hotfixes/], the changes to the underlying mail libraries actually broke specific server configurations. If your site suddenly stopped sending mail after the update, you’re likely hitting a bug with the PHPMailer implementation. The core team updated the “Hotfix” plugin specifically to address this until the next maintenance release.
Fixing the Adjacent Post Infinite Loop
Another “fun” surprise was a change to adjacent post navigation. Some developers who were using the get_{$adjacent}_post_where filter started seeing infinite loops that would crash the server. This happened because of an unpublicized change in how the query is structured. If you’re running custom filters on post navigation, you need to be extremely careful with how you’re returning your WHERE clauses 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 );The Lesson Learned
Look, the takeaway here isn’t that you shouldn’t update. It’s that you shouldn’t update blindly on a production site without a staging environment. WordPress is a massive ecosystem, and even small “performance” tweaks can have massive ripple effects. If you’re stuck, the “Hotfix” plugin or the “Load Combined Core Block Assets” plugin are your best friends right now. Use them as a bridge until 6.9.1 lands.
This stuff gets complicated fast. If you’re tired of debugging someone else’s mess and just want your site to work without the Friday afternoon panic, drop my team a line. We’ve probably seen it before.
Are you still seeing weird CSS issues on your classic theme after these hotfixes, or did the plugins clear it up for you?
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