Make sure that (simplevalues excepted) for every subnode mentioned in the language grammar, there is a top-level grammar rule defining that subnode.

For example, when analysing the xml language grammar we found this rule

<TypeDeclaration>
	...
	<modifiers nodelist="IExtendedModifier"/>
	...
</TypeDeclaration>

But we didn't find neither nor in the XML language grammar definition. (Or maybe we didn't look well enough?)

More on this issue can be found in point 4.F and 3.a.8 of the file metamodel/experiment1.txt