One of Terry Tao's recent results shows that an equation that he calls the "averaged Navier-Stokes equation" can have solutions that "blow up" in finite time, starting from a perfectly nice initial condition [0]. Presumably the writer is referring to this, as such solutions would not be very nice to have if they are actually physically realizable, for example by an airfoil.

[0] https://terrytao.wordpress.com/2014/02/04/finite-time-blowup...