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 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)