Can you post more info about this ? Stack trace ?
I understand that when there is more than just one version of the same class deployed on JBoss, it simply gets the first one deployed unless you have defined a different approach. So, if you have the same lib/class deployed more than once, the problems becomes getting to know if your app is calling the wrong version.
If you want to check what version of a class JBoss is actually using, you can use the jmx-console to ask what class loader is getting it and also from what archive (look for LoaderRepository in the main screen).