How to reproduce in Unity 5:

1. Open the attached project
2. Open Demo scene
3. The cube in the scene is a rigidbody
-The rotation constrains for X and Z are checked, Y axis is left unchecked
-The cube should be able to rotate around the vertical axis only.
3. Run the scene.
4. Press and hold left mouse button while the cursor is over the cube. This applies a little force at the given position.
5. The cube should start to rotate.
6. Notice, that the axis, it rotates around, is its local Y-axis, not the world Y-axis

Captured reproduction of the bug: http://screencast.com/t/SVvKFFmw

According to the documentation : "Stops the Rigidbody rotating around the world X, Y and Z axes selectively."