Well, the first upgrade (for me) went fairly poorly. The site continued to work just fine but the authors could not post or upload new media. Clicking the media button on a post did nothing and the media manager “Add Media” option did not load the Media drag & drop area.
Diagnosis involved opening up the WordPress backend in Chrome, clicking Add Post and then using right click to “Inspect Element”. In the lower right, there was an javascript error indicator so I clicked that and it was revealed that the following url (of which names have been changed to protect the innocent) was giving a 404 instead of a set of scripts all concatenated together:
http://unknown-site.com/wp-admin/load-scripts.php?c=0&load[]=jquery,utils,plupload,plupload-html5,plupload-flash,plupload-silverlight,plupload-html4,plupload-handlers&ver=3.5
What this means is that WordPress is supposed to consolidate all the scripts together and then deliver them via that load-scripts.php call instead of delivering them one at a time. I believe this is something that was available in an earlier version of WordPress but I’m not sure if this site was utilizing that feature prior to the upgrade (and in the midst of an outage where all the authors were sitting doing nothing and waiting for the site to start working again – there wasn’t really time to do that kind of analysis).
Bottom line is – either 3.5 turned this feature on and it is not compatible with this site’s configuration/plugins or this feature was in use before and the upgrade has now broken it. (More updates should give more insight.)
However, if you are having this issue there is an easy work around – just add this line to your wp-config.php to disable script consolidation:
define( 'CONCATENATE_SCRIPTS', false );
Time to go start performing more updates to see if this is a real issue or simply an isolated perfect storm situation.
UPDATE: Thanks to Ralph’s comment below, I now know that there is an incompatibility with BPS (Bullet Proof Security) and WordPress 3.5. However, the plugin author has released a new version (.47.7) that corrects this issue. If you encounter this – try updating BPS and then force it to update the htaccess rules.
UPDATE 2: I’ve encountered this same issue on another site, but disabling all plugins did not resolve it. It appears to be caused by the theme. The fix described above does solve the issue and I have not had a chance to look at the theme. It is a commercial theme – so I fear this issue may be more widespread than originally thought.