Steps to repro:

1. Open attached project and t1 scene on Windows;
2. Select Terrain gameobject in the Hierarchy;
3. In the Inspector go to 'Place Trees' tab;
4. Click Edit Trees - Add tree;
6. In the Project folder open Assets -> Free_SpeedTrees;
7. Drag 'Broadleaf_Desktop.spm' to 'Tree Prefab' field of 'Add Tree' popup in such way that this prefab is also raycasted on terrain in Scene view (see attached video for clarity).

Expected results:
- Tree prefab is dragged to 'Tree Prefab' field of 'Add Tree' popup;
- Tree prefab, dragged to 'Add Tree' field isn't rendered in Scene view on terrain.

Actual result:
Tree prefab is rendered on Terrain. See attached video.
Regression introduced in Unity 2017.1.0a4

Notes:
- If selected in Scene view, this dragged tree prefab has no reference to Hierarchy and cannot be removed by undo;
- Reproducible in 2017.1.0a4; not reproducible in 2017.1.0a3, 5.6.0f1, 5.5.2p1;
- Reproducible only on Windows 10, not reproducible on OSX 10.12.3;
- Also happens when dragging TreeCreator prefabs to 'Add Tree' popup and other prefabs to 'Add Detail Mesh' popup.