Let's say that you want to move the leg up. Although the clearance around the ball joint is really small, it can be done: Now I want to move the leg outward. Again, no problem: Now here's the problem: I want to move the leg down again, but with the leg slightly outward. Well, you can, for a bit: This is the stopping point: if you pull the leg down further, the clearance isn't enough around the ball joint. Although you might get past it, it will generate stress in the joint You may be thinking that the force applied to get past this rotation is nowhere near to be able to create stress marks: I thought so, after all, the plastic doesn't feel rigid and even some of the transformation steps require to bend the plastic a bit, right? Well... As you can see the bridge between the ball and the waist piece already has a big stress mark. The plastic around this area doesn't have the same properties as the hinges that connect the torso to the wings, for example. It won't become bigger or break with limited movements or by transforming the figure, but it's a design flaw that could have been avoided by lowering the ball joints a few millimeters or by cutting plastic around the socket sculpt. I checked the other leg afterwards: Out of the box it didn't have stress marks. Did a movement using both axis at the same time like in the photos and got the stress mark.