After updating to 3.5 this happens:
When pushing the update button on a post I get a 404 error and get sent to my 404 handling
Sometimes a small change like adding the letter X works, switching to Text mode and replacing an entire section of text has worked once as well.
I've went trough the full troubleshooting routine including deleting all plugins and forcing WP to deactivate them. I also reinstalled wp-admin and wp-includes manually. The problem persists with custom theme and twentyten and twentyeleven.
I verified .htaccess and got only the original wordpress statements plus a .net -> .com redirection
I've added these to wp-config.php
Java-Script console reports as first line when it fails
POST (url)/wp-admin/post.php [HTTP/1.1 404 Not Found 874ms]
and "302 Temporarily moved" the few times it was successful