A fresh install of homebrew on OSX, or any other brew installation;
Installation hangs with the error:
error: Not a valid ref: refs/remotes/origin/master
fatal: ambiguous argument 'refs/remotes/origin/master': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
rm -fr $(brew --repo homebrew/core) # because you can't `brew untap homebrew/core`
brew tap homebrew/core