While deploying some MDB Unit Tests today, came across the following error (sporadically): java.lang.RuntimeException: Failed to download and/or install client side AOP stack I've also seen other variations, reporting CNFE for at least 3 different classes (I've since thrown away these stack traces). The underlying cause is, however, consistent when the error is thrown: Caused by: java.util.ConcurrentModificationException | at java.util.TreeMap$PrivateEntryIterator.nextEntry(TreeMap.java:1031) | at java.util.TreeMap$KeyIterator.next(TreeMap.java:1058) | at org.jboss.mx.loading.LoadMgr3.beginLoadTask(LoadMgr3.java:235) | at org.jboss.mx.loading.UnifiedClassLoader.loadClassImpl(UnifiedClassLoader.java:290) | at org.jboss.mx.loading.RepositoryClassLoader.loadClass(RepositoryClassLoader.java:441) | at java.lang.ClassLoader.loadClass(ClassLoader.java:251) | at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319) | at org.jboss.jms.client.delegate.ClientConnectionFactoryDelegate.org$jboss$jms$client$delegate$ClientConnectionFactoryDelegate$getClientAOPStack$aop(ClientConnectionFactoryDelegate.java:230) | at org.jboss.jms.client.delegate.ClientConnectionFactoryDelegate.getClientAOPStack(ClientConnectionFactoryDelegate.java) | at org.jboss.jms.client.ClientAOPStackLoader.load(ClientAOPStackLoader.java:75) | at org.jboss.jms.client.JBossConnectionFactory.createConnectionInternal(JBossConnectionFactory.java:192) I've been duplicating this by running the "ejbthree1123" test a few times, usually I can get it to pop up. But there's at least a couple threads accessing/writing to one Map instance in there somewhere. :) S, ALR View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4109498#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...