Apple is warning people not to use palm rest covers with their MacBook Pro with Retina Display because they can cause the geometry of the device to shift when closed, and cause damage to the screen.
On a support page — as noted by “Macworld UK” (http://macte.ch/StvDK) — Apple says: “To enable thin design, the clearance between the display and the palm rest area is engineered to tight tolerances. Do not use palm rest covers as the added thickness may interfere with the designed resting position of the display. … As a best practice, keep the palm rest clear of any material.”