Renaming or naming assets with a leading underscore invalidates the asset.
The assets will be renamed to "None" and report occupying 1KB in space.
It is no longer possible to interact with an object in this state. Upon restarting the Editor, the asset will have been deleted.
This has been observed if the asset is named / renamed to value other than "_1".
Naming an asset "_1" results in the asset being named according to its type with a numeric value (Ex: Material_0, Material_1).
Result:
Asset is renamed "None;" no longer interactive
Frequency:
3/3
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-40541 in the post.