PetroviskyPetrovskiĭ-Landis solution to the second part of Hilbert 16th problem. They "proved" the existence of a bound for the number of limit cycles of planar polynomial vector fields of fixed degree. Ilyashenko pointed out the mistake.
The problem remains wide open but the basic idea of PetroviskyPetrovskiĭ-Landis ( complexification of real differential equations ) lead to the study of holomorphic foliations.