It would be great if there could be some sort of notification that a layer in an ArcGIS Online map is shared differently than the map and/or rest of layers. We have a map with 30+ layers that we're constantly adding to. If just one of those layers is not shared properly (i.e. with the group or organization), none of the other layers show up. We have to go through and check the sharing of each layer individually to determine where the problem is.
I envision something like the "broken data source" red alert in ArcMap Table of Contents.
