I have been chasing a failure that occurs very very rarely (twice since I started to work on npm few weeks ago).

Concurrent call to Object.create in parallel with object instantiation makes the PropertyListenerManager of the builtin Object prototype to :
1) handle removeNotificationListener when Object.create is called
2) handle addNotificationListener for object literal.

This is causing java.util.ConcurrentModificationException

The list of listeners should be synchronized.