Plan 9 from Bell Labs’s /usr/web/sources/contrib/fernan/nhc98/src/libraries/HaXml/tests/xml-conformance/ibm/not-wf/P78/ibm78n02.xml
<!DOCTYPE root [ <!ELEMENT root (#PCDATA)> <!ATTLIST root att CDATA #IMPLIED> <!--* error in external entity *--> <!ENTITY aExternal SYSTEM "ibm78n02.ent"> ]> <root>&aExternal;</root>