Nested in the sense of being a non-public class in the same source file. For example the following three classes are all in BlaBlaTest.java

class BlaBlaTest {
  public void akkaTest() {
    final ActorRef subscriber = system.actorOf(new Props(Subscriber.class));
    final ActorSystem system = ActorSystem.create("system");
    system.eventStream().subscribe(subscriber, Event.class);  <== throws exception
  }
}

class Event {
}

class Subscriber extends UntypedActor {
  @Override
  public void onReceive(final Object message) {
    System.out.println("Got " + message);
    unhandled(message);
  }
}

This throws a message like:

[ERROR] [06/11/2012 13:31:54.19] [system-akka.actor.default-dispatcher-2] [akka://optimus11/user/$a] error while creating actor
java.lang.IllegalAccessException: Class akka.actor.Props$$anonfun$$init$$2 can not access a member of class Subscriber with modifiers "public"
	at sun.reflect.Reflection.ensureMemberAccess(Reflection.java:95)
	at java.lang.Class.newInstance0(Class.java:366)
	at java.lang.Class.newInstance(Class.java:325)
	at akka.actor.Props$$anonfun$$init$$2.apply(Props.scala:132)
	at akka.actor.Props$$anonfun$$init$$2.apply(Props.scala:132)
	at akka.actor.ActorCell.newActor(ActorCell.scala:488)
	at akka.actor.ActorCell.create$1(ActorCell.scala:506)
	at akka.actor.ActorCell.systemInvoke(ActorCell.scala:600)
	at akka.dispatch.Mailbox.processAllSystemMessages(Mailbox.scala:209)
	at akka.dispatch.Mailbox.run(Mailbox.scala:178)
	at akka.dispatch.ForkJoinExecutorConfigurator$MailboxExecutionTask.exec(AbstractDispatcher.scala:516)
	at akka.jsr166y.ForkJoinTask.doExec(ForkJoinTask.java:259)
	at akka.jsr166y.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:975)
	at akka.jsr166y.ForkJoinPool.runWorker(ForkJoinPool.java:1479)
	at akka.jsr166y.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:104)

However, it all works when class Subscriber is put into a separate source file.