When I was trying to create a menu in MS CRM 2011 ribbon and  I set DisplayMode=”Default” for menuselection I got this error, and when I tried to search this error in I found http://msdn.microsoft.com/en-us/library/gg334378.aspx,

According to SDK :- “You may get the following error “MenuSections can only have children of type MenuItem, Gallery or GroupPopup.” when viewing a Ribbon that has a MenuSectionDisplayMode attribute value set to the following values: Default, Small, Medium, Large, Text, Thin. While according to the RibbonWSS.xsd these are valid values, they should not be used.”

Hope it will help