When I create a git repository for a new project on Drupal.org I don't bother to create a master branch, branches named 6.x-1.x or 7.x-1.x have special meanings and are the ones that we're encouraged to use. However, drupal.org doesn't allow us to [change the default branch][branch-issue] on d.o itself, so even though there may be no branch called 'master', it's still the default branch, so sometimes cloning a repo will fail:
git clone http://git.drupal.org/sandbox/darthsteven/1268648.git
Cloning into 1268648...
remote: Counting objects: 177, done.
remote: Compressing objects: 100% (176/176), done.
remote: Total 177 (delta 103), reused 0 (delta 0)
Receiving objects...