The Lockheed L-188 Electra, notoriously, suffered from early problems with whirl-mode flutter, which caused two Electras to break apart in flight in 1959 and 1960, forcing an expensive redesign of the Electra's engine mounts and killing off Electra sales.
In contrast, the other similar quadturbopropliners of the era (Viscount, Britannia, Il-18, An-10, CL-44, Vanguard) - even those which entered service before the Electra ever flew - had no such problems.
Why?