Maybe this is intentional design, but when attempting to re-calculate the threshold for "Speed" the BlendTree slider gets broken - please see attached screen shot for details.

To reproduce:
- Download the "Mecanim Example Scenes" from Asset Store
- Open "Generic Skeleton Example" scene
- Select "mine_bot" object in Hierarchy
- Open "MineBot" animation controller
- In the Animation Controller window, double-click on "Locotion"
- Select the "Loco" blend tree and a new Motion Field
- Un-tick the option for "Automate Thresholds"
- In the parameter for "Computer Thresholds", select 'Speed'
- Observe that the slider in blend tree is now broken