Steps:

Case 1 - Cannot delete texture
1. Open repro project
2. Delete the texture called "Error" and "Warning"
3. Close Unity and Open Unity again
4. The 2 textures are still there

Case 2 - Resources.Load returns null
1. Open repro scene
2. Hit Play
3. In console, there is a message say "Null" for the "testTexture" which exists under the Resources/Textures folder
4. Change the script to load "Error" or "Warning" still returns null

Reproducible in : 2018.2.0a2, 2018.1.0b9, 2017.3.1p2
Not reproducible in : 2017.2.2f1