Extra file in GHC repo

Hi devs I've started to get this a lot bash$ git rebase origin/master First, rewinding head to replay your work on top of it... error: The following untracked working tree files would be overwritten by checkout: docs/index.html Please move or remove them before you switch branches. Aborting fatal: Could not detach HEAD Removing docs/index.html makes things work again. What am I doing wrong? Simon