Someone reported this to me yesterday and I just hit it myself today:
> arc patch --nobranch D22362 --trace libphutil loaded from '/home/josh/workspace/github.com/joshuaspence/dotfiles/modules/phabricator/libphutil/src'. arcanist loaded from '/home/josh/workspace/github.com/joshuaspence/dotfiles/modules/phabricator/arcanist/src'. Config: Reading user configuration file "/home/josh/.arcrc"... Config: Did not find system configuration at "/etc/arcconfig". Working Copy: Reading .arcconfig from "/home/josh/workspace/git.mydomain.com/eng/site/.arcconfig". Working Copy: Path "/home/josh/workspace/git.mydomain.com/eng/site" is part of `git` working copy "/home/josh/workspace/git.mydomain.com/eng/site". Working Copy: Project root is at "/home/josh/workspace/git.mydomain.com/eng/site". Config: Did not find local configuration at "/home/josh/workspace/git.mydomain.com/eng/site/.git/arc/config". Loading phutil library from '/home/josh/workspace/git.mydomain.com/eng/site/support/flarc/src'... >>> [0] <conduit> differential.querydiffs() <bytes = 76> >>> [1] <http> https://phabricator.mydomain.com/api/differential.querydiffs <<< [1] <http> 1,150,089 us <<< [0] <conduit> 1,153,731 us >>> [2] <conduit> conduit.connect() <bytes = 555> >>> [3] <http> https://phabricator.mydomain.com/api/conduit.connect <<< [3] <http> 363,746 us <<< [2] <conduit> 364,390 us >>> [4] <conduit> differential.querydiffs() <bytes = 200> >>> [5] <http> https://phabricator.mydomain.com/api/differential.querydiffs <<< [5] <http> 371,120 us <<< [4] <conduit> 371,851 us >>> [6] <exec> $ git remote show -n origin <<< [6] <exec> 6,242 us >>> [7] <conduit> repository.query() <bytes = 244> >>> [8] <http> https://phabricator.mydomain.com/api/repository.query <<< [8] <http> 361,050 us <<< [7] <conduit> 361,681 us >>> [9] <exec> $ git diff --no-ext-diff --no-textconv --raw 'HEAD' -- >>> [10] <exec> $ git ls-files --others --exclude-standard <<< [10] <exec> 119,503 us <<< [9] <exec> 132,776 us >>> [11] <exec> $ git diff-files --name-only <<< [11] <exec> 38,095 us >>> [12] <exec> $ git show -s --format='%H' '07729d85436c4b852a114683dbf965114531c2a6' -- <<< [12] <exec> 3,047 us >>> [13] <conduit> differential.query() <bytes = 192> >>> [14] <http> https://phabricator.mydomain.com/api/differential.query <<< [14] <http> 400,084 us <<< [13] <conduit> 400,470 us >>> [15] <conduit> differential.query() <bytes = 453> >>> [16] <http> https://phabricator.mydomain.com/api/differential.query <<< [16] <http> 421,317 us <<< [15] <conduit> 421,872 us >>> [17] <exec> $ git show -s --format='%H' '' -- <<< [17] <exec> 4,613 us >>> [18] <exec> $ git show -s --format='%H' '' -- <<< [18] <exec> 4,428 us >>> [19] <exec> $ git show -s --format='%H' '' -- <<< [19] <exec> 4,556 us >>> [20] <exec> $ git show -s --format='%H' '' -- <<< [20] <exec> 4,863 us >>> [21] <exec> $ git show -s --format='%H' '' -- <<< [21] <exec> 10,936 us >>> [22] <exec> $ git show -s --format='%H' '' -- <<< [22] <exec> 4,050 us >>> [23] <exec> $ git show -s --format='%H' '' -- <<< [23] <exec> 3,717 us >>> [24] <conduit> differential.query() <bytes = 453> >>> [25] <http> https://phabricator.mydomain.com/api/differential.query <<< [25] <http> 416,275 us <<< [24] <conduit> 416,904 us >>> [26] <exec> $ git show -s --format='%H' '' -- <<< [26] <exec> 4,914 us >>> [27] <exec> $ git show -s --format='%H' '' -- <<< [27] <exec> 4,215 us >>> [28] <exec> $ git show -s --format='%H' '' -- <<< [28] <exec> 6,646 us >>> [29] <exec> $ git show -s --format='%H' '' -- <<< [29] <exec> 4,487 us >>> [30] <exec> $ git show -s --format='%H' '' -- <<< [30] <exec> 6,309 us >>> [31] <exec> $ git show -s --format='%H' '' -- <<< [31] <exec> 4,538 us >>> [32] <exec> $ git show -s --format='%H' '' -- <<< [32] <exec> 4,266 us >>> [33] <conduit> differential.query() <bytes = 219> >>> [34] <http> https://phabricator.mydomain.com/api/differential.query <<< [34] <http> 387,173 us <<< [33] <conduit> 387,789 us >>> [35] <exec> $ git show -s --format='%H' '' -- <<< [35] <exec> 4,251 us >>> [36] <conduit> differential.query() <bytes = 804> >>> [37] <http> https://phabricator.mydomain.com/api/differential.query <<< [37] <http> 716,864 us <<< [36] <conduit> 717,468 us [2015-08-13 00:50:09] EXCEPTION: (Exception) You may not set new credentials after authenticating conduit. at [<arcanist>/src/workflow/ArcanistWorkflow.php:222] arcanist(), flarc(), phutil() #0 ArcanistWorkflow::setConduitCredentials(array) called at [<arcanist>/src/workflow/ArcanistWorkflow.php:600] #1 ArcanistWorkflow::buildChildWorkflow(string, array) called at [<arcanist>/src/workflow/ArcanistPatchWorkflow.php:944] #2 ArcanistPatchWorkflow::applyDependencies(ArcanistBundle) called at [<arcanist>/src/workflow/ArcanistPatchWorkflow.php:456] #3 ArcanistPatchWorkflow::run() called at [<arcanist>/src/workflow/ArcanistPatchWorkflow.php:397] #4 ArcanistPatchWorkflow::run() called at [<arcanist>/scripts/arcanist.php:382]