This has bitten me more than a couple of times and it wastes nearly an hour of my time every time it happens.
Essentially if anything goes wrong part-way through running make-wmf-branch the script retries the failed command 5 times then gives up. It is then very difficult to get the branch set up correctly as there is no way to start over and no way to resume. Very horrible experience.
make-wmf-branch should record each step in a log of some kind. When something goes wrong we can abort, manually fix whatever was the cause, and finally re-run make-wmf-branch and use the log from the previous run to get back to where we left off.