I see this all the time with LV 8.6. I can't remember if LV 8.5 did it. I believe reading somewhere else that it is a bug. I don't know if it is truly related to having a RT target, even without running a VI on it, or just a problem with the Project explorer. But basically Autosave failed for some reason. If you click OK, things move on and you don't see it come up again as long as your project is open. Though if you do nothing in the project and try to close it, it will warn you that a project attribute changed.
Basically, it's a nuisance bug, but not a problem.