• References to non-instantiable classes
• References to classes that do not extend required class
• References to classes with inappropriate scope
• Empty tag and attribute values
• 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:

<web-app>
  <filter-mapping>
    <filter-name>CustomFilter</filter-name> // Error: either servlet-name or url-pattern child tag should be defined
  </filter-mapping>
  <servlet-mapping>
    <servlet-name>CustomServletName</servlet-name>
    <url-pattern>/ baseUrl</url-pattern> // Error: URI-pattern must not contain white spaces
  </servlet-mapping>
</web-app>