When running arc diff the following error appears in the console before the EDITOR (in our case vim) opens.
[2015-05-14 05:35:46] ERROR 2: sprintf(): Too few arguments at [<path>/arcanist/src/workflow/ArcanistWorkflow.php:1024] arcanist(head=master, ref.master=f00d4219ddab), phutil(head=master, ref.master=f06c0b71f369) #0 sprintf(string, string) called at [<arcanist>/src/workflow/ArcanistWorkflow.php:1024] #1 ArcanistWorkflow::requireCleanWorkingCopy() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:640] #2 ArcanistDiffWorkflow::runDiffSetupBasics() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:430] #3 ArcanistDiffWorkflow::run() called at [<arcanist>/scripts/arcanist.php:387]
arc appears to continue and opens the editor, but for us, the template in the EDITOR does not display properly.