I'm using OpenJPA in OSGi environment (namely: JBoss Fuse 6.1). We had nasty NullPointerException in Felix when trying to load source code during initialization of class metadata. This line returned duplicate entity classes - half of them were from old revision of a bundle and half - from new revision.
I tracked down the problem to the point, where org.apache.openjpa.enhance.PCRegistry#register() was invoked as part of constructing new instances of entity classes (static initialization injected during class enhancement). Although _metas is instance of ConcurrentReferenceHashMap with weak keys, the keys (entity classes) had other hard references too. Here's a list of the hard refs I got from simple heap dump:
so on every bundle refresh we have next bunch of classes in _metas...