Add your own Menus and sub menus as you see fit.  Run the application out of VS2010.  Notice that when you run the page you created and pass the mouse over the menus the menus drop down and slide out normally.

In the browser (IE8) go to the Tools | Internet Options menu.  When the Internet Options dialog box comes up, click the Security tab.  Select the Trusted Sites zone and click the Sites button.  Uncheck the "Require server verification" check box and add the http://localhost to the Websites list.  Click Close then click OK on the Internet Options diaolog box.

Pass your mouse back over the menus and you should get a javascript error that shows in VS2010.  In the Telerik.Web.UI.WebResource_2.axd?... file you get this error in this function: " 'type' is null or not an object "

 

Sys.UI.DomEvent=function(e){var a=e,b=this.tyle=a.type.toLowerCase();this.rawEvent=a;this.altKey=a.altKey;

where    b=this.tyle=a.type.toLowerCase()   is hghlighted in yellow showing the statement that failed

Go ahead and click the Continue button on the error dialog until the error clears and passes execution on to the browser.  Now refresh your browser by hitting F5.  Pass your mouse over the menus and the error no longer happens.

Can anyone tell me why this might be happening?

Ralph