AA191 in May 1979 - number 1 engine separation from a DC-10 on takeoff at ORD, killing all 271 on board and two on the ground. The subsequent investigation found the probable cause was damage to the engine pylon caused by bad maintenance practice, namely unauthorised use of a fork lift truck to lift the engine during an earlier engine change at AA's Tulsa maintenance facility.
Good summary on
Wikipedia.
C.