Steps to repro:

OSX:

1. Open attached project and 't10TreeShadows' scene;
2. Go to Edit -> Project Settings -> Player;
3. Uncheck 'Auto Graphics API for Mac' checkbox;
4. Drag Metal on top of the list;
5. Observe console.

Windows:

1. Open attached project and 't10TreeShadows' scene;
2. Go to Edit -> Project Settings -> Player;
3. Uncheck 'Auto Graphics API for Windows' checkbox;
4. Click '+' button and add OpenGLES2 platform and OpenGlCore;
5. Drag 'OpenGLCore' on top of the list;
6. Drag 'OpenGLES2' on top of the list.

Actual result:

'Internal: Possible leak. All textures should be removed from IdMap when destructed using RemoveTexture(TextureID texid).'
is thrown in the console. See attached gif video.

Expected result:
Graphics API should be switched without errors.

Notes:
- Reproducible only on OSX 10.12.2 and on Windows 10;
- Reproducible in 5.6.0b6, 5.6.0b5, 5.5.0p4, 5.4.3f1.
- Reproducible only when switching Graphics API to Metal;
- Not reproducible in Standard Assets scene.