On p.47 of the Player's Guide it states: "A character may not enter an advanced career until he has achieved at least Rank 2."

I like this rule... but I can't find it anywhere in the core set books or errata.

Was this a stealth errata added to the Player's Guide?

Yes. Looks like this rule was added in the PG, along with the Specialist trait always being an incompatible career trait. Neither rule appears in the core or the FAQ/errata. I don't know if it's a stealth errata, or if the radar is just broken.

-Thorvid