Recently upgraded to 3.5
All the formatting options for 'Add Post' disappeared - the buttons in the grey border at the top of the 'Visual' box. This appears in IE9 and FF18.
I deactivated all plugins and switched to twenty-eleven, and this had no effect.
define (SCRIPT_DEBUG', true);to wp-config.php and the buttons reappeared.
Note: I have a similar problem with widgets, where I cannot drag them to the sidebar unless the debug is active.