The two that come to mind are splitting epics in Set and taking the Skel of a category. Surely there are lots of other interesting (and maybe upsetting) places where this comes up.



Where are some interesting places where the axiom of choice crops up in category theory?
Using the usual definition of "functor," almost any functor
constructed using only universal properties requires the axiom of
choice. For instance, if a category C has products, then one wants a
"product assigning" functor C×C → C, but in order to define this you
have to *choose* a product for each pair of objects. If C is a large
category, then you need an axiom of choice for proper classes.


However, this sort of thing is arguably not a "real" use of the axiom
of choice. It's more accurate to say that in the absence of the axiom
of choice the usual definition of "functor" is not sufficient, and one
must use [anafunctors](http://ncatlab.org/nlab/show/anafunctor)
instead. Proving that fully faithful + essentially surjective =
equivalence is the same. Most often in category theory when we want
to "choose" something, that thing is in fact determined up to unique
isomorphism (though not uniquely on the nose) and in that case using
anafunctors is sufficient to avoid choice.


The axiom of choice does, however, come up in the study of particular
properties of the category Set. One interesting consequence of the
fact that epics split in Set is that *all functors defined on Set
preserve epics*. I think this is an important part of Blass' proof
that the existence of nontrivial (left and right) exact endofunctors
of set is equivalent to the existence of measurable cardinals.
Another interesting consequence is that Set is its own "ex/lex
completion."


