Tim rolled back wmf12 after a nasty bug last night: https://bugzilla.wikimedia.org/show_bug.cgi?id=46397
Our fix we deployed to test2 didn't fix it: https://gerrit.wikimedia.org/r/#/c/55086/
We're still diagnosing/etc.
So, we're staying on wmf11 for now (except on test2, which is running the broken fix in wmf12).
Please ping me or Robla with any questions.
Greg