STR:
1. Open user attached project
3. Select the "Terrain" GameObject in the scene.
4. Click Play.
4. During runtime, press the "Paint Texture" icon.
5. Click "Edit Textures..." then "Add Texture...".
6. Select a Splat Texture (for example "Default-Checker") and click "Add". The terrain will now have five textures in total.
Notice there will be an assertion error in console window.

Not reproduced with: 5.3.5f1, 5.4.2f2
Reproduced with: 5.5.0b9, 5.5.0b10