Jboss5.1GA has a wrong default setting. It means different EARs will cross-reference by default setting. If you have two EARs. One EAR use MAT-1.0.jar, by another use MAT-1.1.jar. The first started EAR will correct, but the last started EAR will wrong.
Please refer to https://jira.jboss.org/jira/browse/JBAS-7371
Here, I copy important content
This seems related to JBAS-7032. You should enable isolated ear deployment either through the use of classloader configuration in jboss-app.xml or by setting "isolated" = true in JBOSS_HOME/server/servername/deployers/ear-deployer-jboss-beans.xml in the following section:
<bean name="EARClassLoaderDeployer" class="org.jboss.deployment.EarClassLoaderDeployer">
<!-- A flag indicating if ear deployments should have their own scoped
class loader to isolate their classes from other deployments.
-->
<property name="isolated">false</property>
</bean>
Starting JBoss AS 5.2 Beta, this property will be set to true by default, so you won't run into such issues.