Skein was enabled. The file below was not checked or committed.
Repro (based on how I made it happen):
-Enable Skein (have 2 users active at the same time)
-Add a texture file and convert it into a sprite.
-Delete the texture.
-Press force delete.
-Asset won’t be deleted and the following message will appear:
EDIT: Realized that I made a sprite from that texture and since sprites are not exposed, that was the one reference that the popup referred to, so I am adding this to the repro steps. Still should let me delete it.