I recently ran into a real mess when trying to persist a typed singleton object in a Flex Mobile application via the PersistenceManager class.
DevGirl Holly Schinsky has a great blog post on managing data in a Flex Mobile application, and I was specifically using her instructions on persisting typed objects. The basics are this:
1.) Register the class alias
2.) Get an instance of the PersistenceManager class
var persistenceManager : PersistenceManager = new PersistenceManager();
3.) Save the object to be persisted
4.) Re-load the object at a later time
Everything works totally fine UNLESS the object you're persisting has a hidden internal class - a class who's scope is internal to the model object class itself. This is often the case for singleton classes in AS3 - since there's no way to make a private constructor the best-practice for singletons in AS3 is to make the constructor take an instance of an internal class object so that no external classes can call the constructor.
I suspect the error is during de-serialization - since the serializer can't make an instance of your singleton model's internal class, it can't make the model class itself at all, and de-serialization fails.
For now I'm just removing the internal classes from my singleton. It means it's no longer really secure, but I'll take the easy data persistence and built-in serialization of the PersistenceManager over having to do the serializing/de-serializing myself.
I suspect this would be an issue with any AMF serialization in Flash - I bet these types of locked singleton's can't be persisted across the client/server or to a regular LSO or anything like that either.