The nodes you see belong to the scene view.
Behavior introduced in r20859 ( issue 0002229 ).

It is not possible to save those nodes (no extension to save) because the MRML scene of the storable nodes scene doesn't contain a DataIOMAnager (vtkMRMLVolumeArchetypeStorageNode::InitializeSupportedWriteFileTypes fails).

Isaiah, it would be good to have a Step by Step procedure to reproduce (I failed at reproducing at first).

Work, save, close, re-open, work, save, close, etc. Nothing unusual. It has happened for multiple separate scenes over past three days. These are mainly scalar nodes, except for one diffusiontensor node. I'm running 9/10 nightly, but Sonia confirmed this bug to me and she is using 9/19 nightly. I'm not using scene views at all.

Isaiah, are you loading an existing scene or creating a new one. If first can you provide the scene with the data?

Actually, if those nodes were in the scene at some point (and in the master scene view) this may be the expected new behavior. Save dialog exposes storable nodes that are present in scene views but not in the main scene. You don't have to save them though.

The nodes were in the scene and then I deleted them. I had loaded and saved the scene several times before deleting these nodes.

As far as I know, there were no scene views involved because I never created any.

It is already difficult to be certain everything is saved in the correct directory, correct format, and with a correct and unique name. I think it would be very unfortunate to require users to mentally disambiguate multiple versions/views of data from this long list in order to make sure the changes are properly saved.

Also, in cases where the format menu is not selectable, it's unclear what is the purpose of displaying these rows if the nodes/ghosts cannot actually be saved.

For information a sceneview node is automatically created when you save the scene.
So when you load it again, you have a scene view.

I should have been more precise: I never explicitly created any extra scene view.

As an aside, this explains something else: every time I reload the scene, there is another pair of "view" and "camera" nodes. Saving and reloading a few times means that I now have 8 or 10 of these extra nodes.

For the aside, the sceneview node doesn't explain why "view"s and "camera"s are duplicated.
This is explained because the scene is "imported" and not "connected". With "connect", the existing scene is cleared before "import". Which prevents duplication.
As of last week, the "view"s shouldn't be duplicated anymore because they are now singletons. Maybe cameras should be singletons as well...

Fixed by stopping saving storable nodes in the Main Scene View since they either in the main scene or are deleted on purpose.

Isaiah, if you delete those extra nodes and save the scene they should not come back.