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]
```