Namespace prefix 'xsd' is not defined.
Scenario: You are saving a page you have edited.
This distressing error can occur when you are using IE9 and the browser is set to IE9 Standards and you try to save a page you have edited. It may also occur in other UI components such as the people picker. It could be a side-effect of branding changes - I haven't so far seen this using the Out-Of-Box branding.
The problem seems to be to do with the ribbon control and some AJAXy post-back type thing on the page - I don't understand all this client-side programming. Whatever the precise cause, it is to do with the browser compatibility mode.
There are two possible solutions to this problem, depending on whether you want a quick fix or a permanent solution.
1) Set your browser to use IE8 standards - you can do this by using F12 to get the IE developer tools panel and setting Document Mode to IE8 standards in the developer tools menu/ribbon thing.
2) Add the meta tag: <meta http-equiv="X-UA-Compatible" content="IE=8"/> to your master page. This will force the browser to use IE8 Standards.
Unfortunately you lose the benefit of the IE9 rendering engine which why it might be preferable to use the first method as and when required, rather than forcing all IE9 users to effectively downgrade to view your pages.