In this paper, we consider the Cauchy problem for one nonclassical, third‐order, model partial differential equation with nonlinearity |u|q in
R3. We show that for q ∈ (1,3], there are no local‐in‐time weak solutions of the Cauchy problem with initial functions u0(x) from a sufficiently wide class, whereas for q > 3, such a solution exists.