The demonstration video has 210 Mb, that's too large to upload here. It can be downloaded from Google drive at https://drive.google.com/open?id=0BySMOfRvmMEgZE0wM1AwcUdkWjQ. Open it with VirtualDub because that's the only program I know that can reactively and fluidly step through single frames both forward and backward (the frame rate is 2000 fps). As the red pressure ball is sliding along the thread segment, it crosses a vertex from time to time and then the normal vector first bends and then suddenly jumps about 30°. Similar to a sawtooth wave form. The jump happens in a single timestep, no matter how low the simulation time step is set. Those jumps propagate across the model and when the total c(lamp)force gets higher and penetration gets deeper, they are causing the whole simulation to explode. Welding normally clamped objects like washers and covers to the worldbody makes the system more stable as the waves cannot propagate any more. Another problem can be seen at the transit between the grey and the violet thread segments. There are two normals vectors then. I don't know if the changes at that transition are smooth enough to prevent explosions because the sawtooth-like jumps are now also there. Maybe the mesh transition will cause unstabilities too at higher rotation speeds. Those two normals wouldn't be there if MuJoCo supported a concave collider. There are so many other options which only have subtle influences. A concave collider would also make mesh modelling more easier.