Steps to reproduce:
1. Open any Unity project
2. Create a new GameObject in the Hierarchy
3. Add a timeline to a GameObject via the Timeline Editor window
4. Drag and drop 'GameObject' to the Project View to create a prefab
5. Make any changes to the GameObject
6. Make sure Timeline is in Preview mode
7. Drag and drop 'GameObject' on top of the GameObject prefab in the Project View

Actual behavior: changes were applied to a GameObject which was in Preview mode which cannot be reverted
Expected result: drag and drop shouldn't apply changes to a prefab when in Preview mode

Reproducible with: 2017.1.0f3, 2017.2.0b3

Fixed in: 2017.1.2p4, 2017.2.0p3, 2017.3.0b9