A user reported this to me:
```
[22-Apr-2015 11:29:22 Australia/Sydney] [2015-04-22 11:29:22] EXCEPTION: (RuntimeException) Undefined variable: revision at [<phutil>/src/error/PhutilErrorHandler.php:210]
[22-Apr-2015 11:29:22 Australia/Sydney] arcanist(head=master, ref.master=a9ebe1cb4ffc), phabricator(head=master, ref.master=8c39885076b5), phlab(head=master, ref.master=e7d4da4d8b26), phutil(head=master, ref.master=aaee4009e20d)
[22-Apr-2015 11:29:22 Australia/Sydney] #0 PhutilErrorHandler::handleError(integer, string, string, integer, array) called at [<phabricator>/src/applications/differential/controller/DifferentialChangesetViewController.php:179]
[22-Apr-2015 11:29:22 Australia/Sydney] #1 DifferentialChangesetViewController::handleRequest(AphrontRequest) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:196]
[22-Apr-2015 11:29:22 Australia/Sydney] #2 AphrontApplicationConfiguration::processRequest(AphrontRequest, PhutilDeferredLog, AphrontPHPHTTPSink) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:121]
[22-Apr-2015 11:29:22 Australia/Sydney] #3 AphrontApplicationConfiguration::runHTTPRequest(AphrontPHPHTTPSink) called at [<phabricator>/webroot/index.php:19]
```
It seems to happen when reviewing a differential from `arc diff --preview`, I //think//.