Race condition in switchboard
https://gitlab.com/mailman/mailman/blob/master/src/mailman/core/switchboard.py#L209 has a race condition: if for some reason multiple runners are working on the same file at the same time, it can happen that the os.rename fails. This triggers the error handling way higher up the stack in src/mailman/core/runner.py (line 168 I think), which shunts the entire message.