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