What to do when Git decapitates you (28 dec 13)
Sometimes, Git says you are in 'detached head mode'.
A good explanation of this is here: http://stackoverflow.com/questions/10228760/how-to-fix-a-git-detached-head
You might be working on
In Scott Chacon's free online Git book, on page 157, is a more comprehensive explanation of detached head: http://git-scm.com/book
P.S. Buy the print edition. Scott is a great guy who explained Git at the Rails conference in 2008, and subsequently gave me free online help: http://www.amazon.com/Pro-Git-Scott-Chacon/dp/1430218339.