Apple announced this week that a number of 13-inch MacBooks may run warmer than normal because they forgot to remove a plastic strip which blocks the rear exhaust vent.
"Some MacBook computers may appear to be running too warm, with the fan running consistently and heat emanating from the top and/or the bottom of the computer," Apple said. "If this happens with your MacBook, check the rear vent of the MacBook to make sure it's not blocked."
The company admits that "some MacBooks may have left the factory with a thin piece of clear plastic covering the rear vent." MacBook owners who find this plastic film covering their notebook's rear vent (where the screen and keyboard almost meet) should "simply remove and discard it."