Full Idea
The non-categoricity of the axioms which Zermelo demonstrates reveals an incompleteness of a sort, ....for this seems to show that there will always be a set (indeed, an unending sequence) that the basic axioms are incapable of revealing to be sets.
Gist of Idea
Non-categoricity reveals a sort of incompleteness, with sets existing that the axioms don't reveal
Source
Michael Hallett (Introduction to Zermelo's 1930 paper [1996], p.1215)
Book Reference
'From Kant to Hilbert: sourcebook Vol. 2', ed/tr. Ewald,William [OUP 1996], p.1215
A Reaction
Hallett says the incompleteness concerning Zermelo was the (transfinitely) indefinite iterability of the power set operation (which is what drives the 'iterative conception' of sets).