ConcurrentModificationException when using multiple windows

When a new browser window is opened to the CMS, java.util.ConcurrentModificationExceptions are logged from within frontend (wicket) code. This implies that multiple threads are accessing the same Page instance. This should not be possible.

08.04.2010 11:08:29 ERROR [org.hippoecm.frontend.JcrObservationManager$JcrListener.process():652] Error occured when processing event
java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
at java.util.HashMap$EntryIterator.next(HashMap.java:833)
at java.util.HashMap$EntryIterator.next(HashMap.java:831)
at org.hippoecm.frontend.plugin.impl.PluginManager.internalGetReference(PluginManager.java:299)
at org.hippoecm.frontend.plugin.impl.ServiceRegistration.notifyTrackers(ServiceRegistration.java:54)