I am attempting to add a few submodules to my .vim/bundles directory, and when I attempt to add this particular repo Git gives me a strange error I’ve never seen before:

Any idea what can cause this?

If I clone the same repo to a test directory (not through the submodule command), it works fine, and creates the expected files.

To fix that error, you should delete the folder with the same path to the submodule inside .git/modules/ directory. This error can occurs when url was incorrect for submodule for the first-time when submodule was added.