An RAF Liberator transport returning from the Casablanca conference crashed on approach to RAF Talbenny in January 1943; the highest ranking casualties were Brigadiers Stewart and Dykes. That's the only incident specifically connected to one of the major conferences that springs to mind, though many others lost their lives in air crashes (including Prince George the Duke of Kent, General Władysław Sikorski (Prime Minister of the Polish Government in Exile), Air Chief Marshal Trafford Leigh-Mallory, Admiral Bertram Ramsay, pioneering aviator Amy Johnson, and the bandleader Glenn Miller).

If you'd like a bit more background on how the Allied leaders travelled, I've got a bit of a work-in-progress on How did world leaders (particularly the Big Three) travel during WW2?.