Plastic dust from the spinning nut on one of the couplers.

I recently fixed another problem with my 3D printer: damage to the z-axis caused by unsecured wires getting in the way of the z-axis limit switch. This caused the nuts to turn entirely out of their housing as the printer reached zero mechanically but couldn't trigger the limit switch to turn the motors off. Read more about the fix here. This is another reason I need to get back to the task of making cable chains and other wire management features to make this printer a little cleaner and better contained.