If you run into Error: Unknown command: cask while trying to use any brew cask command, please run the below command to fix things:
cd $(brew --repo); git fetch; git reset --hard origin/master; brew update
Do not open an issue.
(Technical details: Homebrew Cask's core code has been moved to Homebrew. brew had a bug where it would erroneously report as always being Already up-to-date. This combination made it impossible for Homebrew to find the moved Cask code.)