Running a home Pathfinder game and running a Pathfinder Society game are two completely different things. In a home game, the GM can control what rules items are legal and can become intimately familiar with the options that his characters are using. This means that the GM only needs to know the rules for the characters and the particular NPCs that are involved with the current session. For most sessions, this is all you need, and therefore a prepared home game GM can appear to be the stereotypical all-knowing rules arbiter with a sufficient amount of preparedness. This is impossible in Pathfinder Society. There are currently 34 Campaign Setting books, 38 Player Companions, and 12 hardcover books with material legal in them for Pathfinder Society. While a lot of the books only have a couple of items legal in them, there is still a lot of material available out there – it’s impossible to know it all. And that’s not even counting the massive amount of crunch that’s coming out in the Advanced Class Guide this August. The 6 players at your table could be using any of that crunch for their characters, and they could be using completely different crunch than the 6 players from the last time you GM’d. Some GMs are better at knowing fringe material than others, but nobody’s perfect. No matter who you are, there will be some PFS game where a player pulls out some random build that you’ve never seen before.
The point is, as a PFS GM, the perception of omniscience is overrated because someone will play something that breaks it eventually. It is okay to admit “I don’t know.” In fact, you should admit it every single time you don’t know something. Taking this approach will have a couple good effects on your games.
Philly Pathfinder Society 