<persistence-unit name="PU">
<description>no description</description>
<mapping-file>META-INF/EJB-orm.xml</mapping-file>
<mapping-file>META-INF/Datasource.xml</mapping-file>
<class>b</class>
<properties>
<property name="com.intellij.javaee.persistence.datasource" value="Datasource"/>
</properties>
</persistence-unit>
<exclude-unlisted-classes>true</exclude-unlisted-classes>
</persistence>
_________________________________________---
C:\zzz\aaa\EJB\META-INF\persistence.xml Error:Error:line (13)cvc-complex-type.2.4.a: Invalid content was found starting with element 'exclude-unlisted-classes'. One of '{"http://java.sun.com/xml/ns/persistence":persistence-unit}' is expected.
Usability Problem
Open
Normal
More informative message is required in case of incorrect xml tag position 
<persistence xmlns="http://java.sun.com/xml/ns/persistence">
<persistence-unit name="PU">
<description>no description</description>
<mapping-file>META-INF/EJB-orm.xml</mapping-file>
<mapping-file>META-INF/Datasource.xml</mapping-file>
<class>b</class>
<properties>
<property name="com.intellij.javaee.persistence.datasource" value="Datasource"/>
</properties>
</persistence-unit>
<exclude-unlisted-classes>true</exclude-unlisted-classes>
</persistence>
_________________________________________---
C:\zzz\aaa\EJB\META-INF\persistence.xml Error:Error:line (13)cvc-complex-type.2.4.a: Invalid content was found starting with element 'exclude-unlisted-classes'. One of '{"http://java.sun.com/xml/ns/persistence":persistence-unit}' is expected.