The bearing bolt on older style ART Rope Guides and Positioners (those manufactured between 2004 and 2007) were prone to outer casing deformation when the device was mistreated - mainly due to uncontrolled retrievals in the case of the Rope Guide. ART operating instructions clearly state these devices should never be exposed to any hard knocks.
ART Positioners and Rope Guides made post 2007 have a re-engineered bearing bolt which offers better resistance to deformation of the outer aluminum shell.However, even with these improved bearing bolts NEVER allow your Rope Guide (or Positioner) to drop freely onto any hard surface. This rule pretty much applies to all metallic climbing gear - it may appear bullet-proof but all equipment will fail if mistreated.
The re-engineered bearing bolts are available for retro-fitting if you still own one of these older-style ART devices. Thanks to Christchurch climber, Adam Hartshorne for bringing this to our attention.