To reproduce:
1. Open the "TestScene" in the attached project
2. Select the "Grid" Game Object from the Hierarchy
3. Select the "tile0_0" sprite from the Assets folder and start dragging it into the Scene view
4. Drag the sprite without releasing it towards the edge of the Scene view tab
5. Release the mouse button when cursor leaves the Scene view tab

Desired outcome: The sprite does not appear on the Scene and Hierarchy.
Actual result: The sprite does not appear on the Hierarchy, but is visible on the Scene and in the Game window. Also, it cannot be clicked.

Workaround: In order to remove the "ghost" sprite, the same sprite has to be dragged from the Assets folder into the Scene.

Reproduced with 2017.2.0a4, 2017.2.0b7, 2017.3.0a1, 2017.3.0a4

Fixed in: 2017.2.0b11