I'm hitting this exception when trying to patch a diff with arc patch:
> arc patch --nobranch --trace D21974
...
>>> [0] <conduit> differential.querydiffs() <bytes = 76>
>>> [1] <http> https://phabricator.mydomain.com/api/differential.querydiffs
<<< [1] <http> 1,069,561 us
<<< [0] <conduit> 1,070,107 us
>>> [2] <conduit> user.whoami() <bytes = 117>
>>> [3] <http> https://phabricator.mydomain.com/api/user.whoami
<<< [3] <http> 327,420 us
<<< [2] <conduit> 327,868 us
>>> [4] <conduit> differential.querydiffs() <bytes = 157>
>>> [5] <http> https://phabricator.mydomain.com/api/differential.querydiffs
<<< [5] <http> 4,547,488 us
<<< [4] <conduit> 4,548,173 us
>>> [6] <exec> $ git symbolic-ref --quiet HEAD
<<< [6] <exec> 3,299 us
>>> [7] <exec> $ git rev-parse --symbolic-full-name 'arcpatch-D21974_1'@{upstream}
<<< [7] <exec> 3,304 us
>>> [8] <exec> $ git --version
<<< [8] <exec> 3,108 us
>>> [9] <exec> $ git ls-remote --get-url 'origin'
<<< [9] <exec> 5,374 us
>>> [10] <conduit> repository.query() <bytes = 201>
>>> [11] <http> https://phabricator.mydomain.com/api/repository.query
<<< [11] <http> 362,311 us
<<< [10] <conduit> 362,905 us
>>> [12] <exec> $ git diff --no-ext-diff --no-textconv --raw 'HEAD' --
>>> [13] <exec> $ git ls-files --others --exclude-standard
<<< [13] <exec> 101,006 us
<<< [12] <exec> 110,651 us
>>> [14] <exec> $ git diff-files --name-only
<<< [14] <exec> 47,732 us
>>> [15] <exec> $ git show -s --format='%H' '6d443cae8300ee97c640c8374ea19d36b3ea6cc8' --
<<< [15] <exec> 8,831 us
>>> [16] <exec> $ git symbolic-ref --quiet HEAD
<<< [16] <exec> 3,520 us
>>> [17] <exec> $ git rev-parse --verify 'arcpatch-D21974'
<<< [17] <exec> 4,048 us
Branch name arcpatch-D21974 already exists; trying a new name.
>>> [18] <exec> $ git rev-parse --verify 'arcpatch-D21974_1'
<<< [18] <exec> 3,908 us
Branch name arcpatch-D21974_1 already exists; trying a new name.
>>> [19] <exec> $ git rev-parse --verify 'arcpatch-D21974_2'
<<< [19] <exec> 4,539 us
>>> [20] <exec> $ git show -s --format='%H' '6d443cae8300ee97c640c8374ea19d36b3ea6cc8' --
<<< [20] <exec> 8,100 us
>>> [21] <exec> $ git checkout -b 'arcpatch-D21974_2' '6d443cae8300ee97c640c8374ea19d36b3ea6cc8'
<<< [21] <exec> 288,473 us
Created and checked out branch arcpatch-D21974_2.
>>> [22] <exec> $ git show -s --format='%H' '6d443cae8300ee97c640c8374ea19d36b3ea6cc8' --
<<< [22] <exec> 4,877 us
Downloading binary data for 'phpmailer.lang-br.php'...
>>> [23] <conduit> file.download() <bytes = 169>
>>> [24] <http> https://phabricator.mydomain.com/api/file.download
<<< [24] <http> 337,732 us
<<< [23] <conduit> 338,115 us
[2015-12-10 21:50:05] EXCEPTION: (ConduitClientException) ERR-CONDUIT-CORE: Cannot change a fulfilled promise to rejected at [<phutil>/src/conduit/ConduitFuture.php:58]
arcanist(), flarc(), phutil()
#0 ConduitFuture::didReceiveResult(array) called at [<phutil>/src/future/FutureProxy.php:58]
#1 FutureProxy::getResult() called at [<phutil>/src/future/FutureProxy.php:35]
#2 FutureProxy::resolve() called at [<phutil>/src/conduit/ConduitClient.php:58]
#3 ConduitClient::callMethodSynchronous(string, array) called at [<arcanist>/src/parser/ArcanistBundle.php:688]
#4 ArcanistBundle::getBlob(string, string) called at [<arcanist>/src/parser/ArcanistBundle.php:717]
#5 ArcanistBundle::buildBinaryChange(ArcanistDiffChange, NULL) called at [<arcanist>/src/parser/ArcanistBundle.php:395]
#6 ArcanistBundle::toGitPatch() called at [<arcanist>/src/workflow/ArcanistPatchWorkflow.php:691]
#7 ArcanistPatchWorkflow::run() called at [<arcanist>/src/workflow/ArcanistPatchWorkflow.php:397]
#8 ArcanistPatchWorkflow::run() called at [<arcanist>/scripts/arcanist.php:382]