The errata states: "Page 153, Advancing Skills and Reactions. The descriptive text is misleading in stating that you may only spend a certain number of picks per advancement. The limitations are per advancement, not pick. Thus, a character may increase a maximum of two professional skills, one non-professional skill, two favored reactions, or one non-favored reaction per advancement (provided they have enough picks)."
The constraint on skills seems pretty counter-intuitive. In the first place, using the common assumption that an advancement represents about a year of normal field duty, is it really reasonable that a character could go an entire year without materially improving his competence in more than a couple of areas of professional specialty, and/or a single hobby? In the second place, since advancement picks can't be held over, doesn't the constraint force players to take a lot of skill specialties and/or inflate their favored reaction and/or buy lots of edges, to get rid of the "extra" one or two picks remaining after they've spent on skills?
I'm trying to understand the logic of the errata, and it just flat-out escapes me. The PG as written seems much more reasonable: you can't put more than two picks into a single skill in a single advancement, but there's no overall limit to the number of picks you can spend on skills in a single advancement.