After receiving an NOA and paying the fee, I was anticipating an issue notification within days. Two days after paying the fee, I instead received a Notice of Withdrawal of Allowance.
The uploaded document, "Notice of Withdrawal from Issue Branch (PTOL-67)" shows that the application was withdrawn by the Office under 37 CFR 1.313(b), which could be for one of the following yet-unknown reasons: (1) a mistake on the part of the Office: (2) a violation of 37 CFR 1.56 or illegality in the application; (3) unpatentability of one or more claims; or (4) for interference or derivation.
Public PAIR transaction shows that there is now a non-final rejection. It'll be a few days to find out the exact reason and then plan to act accordingly (my guess is that it is (3), due to a newly discovered prior art, but we'll see).
I understand that this is a relatively rare occurrence. My lawyer said this happens in a small % of allowed cases but I've looked through dozens of cases without finding any, and have only uncovered 2 or 3 via Google.
Any ideas how often this happens, and how I can search out more cases to review?