Summary: By default Git push used to (they've changed it) push ALL your branches, not just your current one. It is pretty much the first thing I've historically reconfigured whenever starting on any new project. This guy didn't do it, and paid the cost.
The issue is pushing to multiple branches.
This is a local developer's Git issue, not a Github issue: http://stackoverflow.com/questions/948354/git-push-default-b... but one they've thankfully changed: http://stackoverflow.com/questions/13148066/warning-push-def...
Summary: By default Git push used to (they've changed it) push ALL your branches, not just your current one. It is pretty much the first thing I've historically reconfigured whenever starting on any new project. This guy didn't do it, and paid the cost.