There seems to be an issue when serializing a type that inherits from PersistentCollection.
I am referring to serialization for transferring objects over the network through Remoting, WebServices or Indigo.
There seems to be a member called collectionSnapshot which is of type ICollectionSnapshot that is public.
the problem is that its implementation - CollectionEntry class is marked internal, and cannot be serialized using the XmlFormatter or the XmlSerializer.
I'm not sure it's needed to be serialized anyway.
To make things short - i tried marking the collectionSnapshot member to [NonSerialized], and now it's working perfectly.
problem is - i don't want to change and recompile NHibernate's the code. Is this a known issue?

No it's not a known issue, please submit a bug report to JIRA.