The following sequence causes Zephyr to produce a non well-formed XML document:

        w.writeStartDocument();
        w.writeStartElement("foo");
        w.writeDefaultNamespace(">:Testing escape:<\"'");
        w.writeEndElement();

output:

        <?xml version="1.0" ?><foo xmlns=">:Testing escape:<"'"></foo>

Namespace URIs need to be escaped too, just like attribute values.
See zephyr/tests/XMLStreamWriter/AttributeEscapeTest for the unit test that produces this error.