It seems that parts with a custom Density below 0.5 cause any BallSocketConstraints connected to them to stiffen. I don’t know when this started happening but I noticed that the physics in my game were acting strangely, and I narrowed it down to the parts’ density and constraints.
This was definitely caused by some update because all the ball constraints in my game have become incredibly hard to move unlike before, which also seriously damages the ragdoll physics (and a new rope feature I made is totally stiff and useless). Please undo whatever caused this.
We enabled a change that could result in something like this on Thursday. The change has been disabled a few minutes ago, can you confirm the current status (whether the problem has been fixed)?
I think zeuxcg already resolved the issue but you could reproduce it by creating two small Parts - size like (0.5,1,0.5) or even (1,2,1) if you minimize its Density, adding an Attachment to their top/bottom side, and connecting them with a BallSocketConstraint. The constraint won’t necessarily act as a static Weld, it will just be more difficult to rotate the joint. I don’t remember the exact details but some smaller parts seemed to try to stiffen into a certain position whereas larger parts just needed more force to move than usual. Reproducing in a zero gravity place might help.