• References to non-instantiable classes
• References to classes that do not extend required class
• References to classes with inappropriate scope
• Tag and attribute values that do not match required pattern (for example, Java Identifiers)
• Tags that do not include required children tags or attributes
• Tags that define objects with duplicate names

Example:

<persistence>
    <persistence-unit name="example-persistence-unit">
        <provider>org.example.SomeRandomClass</provider> // Error: provider class should implement PersistenceProvider interface
    </persistence-unit>
</persistence>