Steps:
1. Create a GameObject and assign any script to it
2. Make a prefab out of this GameObject and left an instance in the Hierarchy
3. Delete a script file from ProjectBrowser
4. Drag Prefab instance to ProjectBrowser
==>
Exception raised, cannot save a new prefab (it is Ok)
Issue:
a) Instance of GameObject is unpacked
b) Unpacked instance is still shown as prefab instnace (blue color and an arrow to open in InContext)
P.S: Reimport of the asset will remove the blue color and an arrow