Here is a playable demo that showcases my problem.

I am importing a relatively simple mesh from Blender 2.5 and adding both a non-kinematic rigidbody and a mesh collider to it. While I know this is not the most efficient way to do things, it seems to be the only way to get the effect I want.

The problem arises when I try to constrain the prefab's rotation to only the Z axis. As soon as I check both the "X" and "Y" boxes (both, not just one) the problem shown in the example starts happening -- note that it seemingly only happens "whenever it feels like it". I have tried bringing down the min penetration in physics settings but it did not solve my problem.

Is this just a side-effect of the combination of settings I am using? Any way around this besides using primitives as children? (I haven't been able to fandangle those into working how I want them to).

It seems like this happens even when I use something like a box collider.

Thanks, Brian