Validation of a simple TEI P5 (TEI Lite) DTD fails consistently with this error message:
Validation stopped at line 3, column 36: Element '_DUMMY_model.resourceLike' was referenced in a content model but never
Very easy to replicate: just generate a TEI Lite DTD with Roma (http://www.tei-c.org/Roma/), create a new XML document with XCE pointing to that DTD and try to validate it. Note that if you create a tei_all.dtd (again using Roma) validation is done without problems.
Using XCE 18.104.22.168 under Linux, also tested this behaviour under Windows using 22.214.171.124.