I wanted to try out helloworld-mdb from http://www.jboss.org/jdf/quickstarts/jboss-as-quickstart/helloworld-mdb/

When you click the big green button "Download" on the left, you're told to use:
git clone git://github.com/jboss-jdf/jboss-as-quickstart.git --branch jdf-2.1.0.Final

But that gives you:

warning: Remote branch jdf-2.1.0.Final not found in upstream origin, using HEAD instead

So obviously the branch is missing.

This should be checked on other places as well.