Before reporting a bug, lets make sure everything is right with your setup.
When reporting bugs, remember that Homebrew-Cask is an independent project from Homebrew. Do your best to direct bug reports to the appropriate project. If your command started with brew cask, bring the bug to us first.
Start by searching for your issue before posting a new one. If you find an open issue and have any new information not reported in the original, please add your insights. If you find a closed issue, try the solutions there. If the issue is still not solved, open a new one with your new information and a link back to the old related issue.
If you did not find your particular bug, before reporting it make sure you have the latest versions of Homebrew, Homebrew-Cask, and all Taps by running the following commands. These will also fix some other issues:
$ cd $(brew --repo); git fetch; git reset --hard origin/master
$ brew untap phinze/cask; brew untap caskroom/cask; brew uninstall --force brew-cask
$ brew cleanup; brew cask cleanup; brew updateRetry your failing command. If the issue persists, go back and pick the appropriate instructions for your problem.