problem switching branches

I was on my master branch, I then checked out a branch ( origin/somebranch ), made no updates but did a few git pulls on the branch over about a week; then made a small change to only a single file & committed & pushed.

Now am trying to go back to my master branch and get:

error: The following untracked working tree files would be overwritten 
by checkout:
         lib/derbyclient.jar
Please move or remove them before you can switch branches.
Aborting

I did not put that jar file there (I edited a single config file), how do I now get back to my master branch?

I do not want to muck up the branch that I am now on by deleting anything. Any ideas how this happened?

Obviously someone put derbyclient.jar there, not sure, but it is supposed to be there so do not want to remove as advised.

any ideas on how I can get back to my master branch and get out of this?