USAir Flight 427
This one involves two different aircrafts which had the same flight number. Both were involved in crazy accidents during the same year. The first was flying on September 8, 1994, and all 132 passengers and crewmates vanished. The second example happened later in 1994. The flight was making a stop in Pittsburgh but the crew lost control of the aircraft and it crashed. It is unclear what caused both flights to crash, and it is certainly mysterious as it involves to planes with the same flight number.
United Airlines Flight 585
On March 3, 1991, Flight 585 crashed. This one is especially surprising because Captain Harold Green and First Officer Patricia Eidson were some of the highest-regarded pilots that always followed procedure. All 25 crew and passengers died.