Stephen Carlson’s proposal has finally hooked me. I believe we should follow “he” instead of “they” in Gal 2:12b, since “he” is witnessed in the best manuscripts. This is Gal 2:11-12:
But when Cephas came to Antioch, I opposed him to his face, because he stood self-condemned; for before certain people came from James, he used to eat with Gentiles. But when [they/he] came, he drew back and kept himself separate for fear of the circumcision faction.
On the traditional (“they”) reading, Peter is already in Antioch, and stops eating with Gentiles when the circumcision faction (the men from James) arrive. On Carlson’s (“he”) reading, Peter came to Antioch with no intention of eating with Gentiles. This would be compatible with the idea that Peter’s behavior was based on treachery, as I believe, with few modifications to Philip Esler’s reconstruction of the Antioch incident. If anything, the “he” reading even better supports this: the pillars (James, John, and Peter) broke their agreement with Paul; Peter later came to Antioch, and knew that men from James were in place to make sure the deal stayed broken.
Carlson’s dissertation, incidentally, is worth reading for well beyond what it teaches about Gal 2:12. See also Richard Fellows’ post, which accepts Carlson’s “he” reading with an alternative scenario.