Objects do not react well to negative scaling with the scaling widget. Once an object hits 0 then pushes into negative values it begins stuttering between small float values until the user stops going towards a negative value and begins dragging towards a positive value, at which point the scale will increase by negative values.
1. Open Editor (any project)
2. Add any mesh to level
3. Switch to scale widget
4. Drag scale widget along an axis towards the object to reduce the value into negative numbers.
5. After it begins rapidly switching between negative values and 0.0 rapidly, begin moving the mouse in the opposite direction.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-12130 in the post.