Steps to reproduce:

1. Download attached DragAndDropError.zip, there are two projects: "DragMe" and "ToThis".
2. Open both of them with same version of Unity.
3. On project "DragMe", select prefab "Bomb", drag it to the scene or hierarchy of other Unity instance with project opened "ToThis".

Expected result: Nothing should happen, Unity disallows dragging directly to scene (first it should be added to assets)
Actual result: Random game object is added, not the same one which was dragged.

Note: Strictly dependent on project structure, changing something makes the bug not reproducible anymore. Produce different results if dragging to the scene or hierarchy.

Regression from: 5.4.0b4 (a7d7a3a2cef4)