I run Jboss 5.1 on Java 5

according to this article:

Case 2. The Utility.class is present both in the application's archive AND server/default/lib. The deployment is non-scoped. The short story: The version of the class available in server/default/lib/utility.jar will be used by the new deployment. The version of the class packed with the deployment will be ignored.

This didn't happened to me. I didn't change anything from JBoss default configuration so I guess my application is not isolated (the application doesn't contain jboss-web.xml file) but still I get an error that indicates that Jboss loaded the application utility class of my application instead of the one of Jboss itself.
It causes the error:

Unexpected error during load of:testdata.SomeAnnotation java.lang.UnsupportedClassVersionError

when removing the relevant jar from web-inf/lib this error gone. So The behavior that I am facing is that the application's class overrides the Jboss server's same class

Is it possible? how can I verify that my application is or isn't isolated?