During routine maintenance, American Airlines has found stress fractures in the engine mounting pylons in two of its Boeing 767-300 aircraft.  These aircraft are in daily service.  The cracks were found at the juncture of where the pylon attaches to the wing.

The manufacturer, Boeing will advise all world airlines to inspect their B767-300 more frequently than the current recommendation of every 1,500 landing/takeoff cycles

To date no cause has been found for this metal fatigue.  Full story here