checking out from branch A to branch B brings along a folder from A. Why

Hi,

I got three branch: master, A, B.

I have a directory in branch A: "/path/to/repo/somefolder" (and that
folder is in the working tree, not working directory), but that folder
doesn't exist in either branch B or master. When in branch A, if I do

    $ git checkout B

that "somefolder" will be brought to branch B (this is weird)

But when I do

    $ git checkout master

then it will not be brought along (this works as expected)

That is weird because AFAIK checkout out to branch B will not bring
any file from the working tree from other branch.

I have tried to updated git from 1.5.4.7 to 2.9.

I am in a CI server so there might be some mis-configuration. What
should I check for? Has anyone ever encountered such a situation?

Yubin