1. Create a Spline object
2. Enter in context and select a knot tangent
3. From the spline inspector, use the slider to set the magnitude value to negative
4. Then, use the slider again to make the value positive

Result
The knot will jump/flicker as we move the magnitude slider

Expected
I don't even think it's right to allow negative value to the magnitude. The direction settings are there for that. If the user uses the Transform tools, the magnitude will always stay positive.