If for example an interface is named using a java reserved word and this 
interface is used within another module, the idl compiler does'nt find that 
symbol.
Look at the following example.

module a {
   interface Package {
   };
};
module b {
   interface Test {
      a::Package doSomething();
   };
};

The idl compiler reports a bug like "symbol b.Test.a._Package not found".