If you get an error "An unhandled exception occurred in the Silverlight Application in SharePoint 2010" when you try to create a document library, list, site or anything in SharePoint 2010 (using new fancy Silverlight control), then it could be that security validation has been disabled Webpart Security Validation under Web Application General Settings in SharePoint Central Administration. Apparently, Silverlight application is unable to connect to the WCF endpoint configured by the product for enabling Client Object Model, if Security validation is set to Off. Enable it, and you should be fine…