This test expects XPST0081. 

"It is a static error if a QName used in a query contains a namespace prefix that cannot be expanded into a namespace URI by using the statically known namespaces."

However, prefix 'hat' is fine here.

I suspect the ocrrect error code is XQDY0027, however, the text in the Appendix F (of XQuery 3.0) for this error code reads:

"In a validate expression, it is a dynamic error if the root element information item in the PSVI resulting from validation does not have the expected validity property: valid if validation mode is strict, or either valid or notKnown if validation mode is lax."

which doesn't mention validate type expressions.

   <test-case name="validate-as-91016">
      <description/>
      <created by="Michael Kay" on="2008-12-01"/>
      <environment ref="hats"/>
      <dependency type="spec" value="XQ30+"/>
      <test><![CDATA[
        import schema namespace hat = "http://www.w3.org/XQueryTest/hats"(:  at "qischema001.xsd" :); 
        validate type hat:hatsize { <hat>banana</hat> }
      ]]></test>
      <result>
         <error code="XPST0081"/>
      </result>
   </test-case>