Enabling the spring resets the "zero"-angle from which the spring calculates its target position. This happens both inside the editor and in the builds.

For example, if the spring's target position is set to 45°, then every time you turn the spring off and on the body will rotate 45° around the joint's axis.

How to reproduce:

1. Open the attached project.
2. Open Demo scene.
3. Press Play.
4. Select the cube.
5. In its HingeJoint component turn "Use Spring" option on.
6. Notice, the cube has rotated 45°.
7. Turn the spring off and on again.
8. goto 6.

or instead of 4,5,6,7,8... just drag'n'drop AutoToggleSpring script onto the cube and observe the same behaviour

Demo: http://screencast.com/t/ydltoyxZ