I came across a git repo recently that output this message with every operation I did:

Your branch is based on 'origin/master', but the upstream is gone.
  (use "git branch --unset-upstream" to fixup)

I was delivering a workshop at the time so I kinda snarled at it and carried on with what I was doing, but later I looked up what is happening. This occurs when a branch is tracking a branch that the git repo doesn't have any information about - the branches to be tracked aren't in the local repo metadata.

In my case, it happened because I had created and then cloned an empty repo for training purposes - so origin/master didn't actually exist yet! I added a quick commit-and-push to my script and hope that I won't be upstaged by this change that came in with git 1.8.5.

Hopefully this post will help someone else to avoid being upstaged or irritated by this as well!