I never get this error, but I received a thumbs down in StackOverflow possible because of it.
If something wrong happens with lockfile creation in line 2440 (https://github.com/defnull/bottle/blob/master/bottle.py#L2440), in finally clause the lockfile variable don't exists, like it is commented in stackoverflow.
My theory is he is using some server adapter which don't support reloader=True.
fix #278: Undefined lockfile variable after failed mkstemp() call.
Thanks Iuri de Silvio
Yes, the questioner was using GAE and so am I, but that's an odd error to get when there's no mention in the docs about GAE not supporting reloader=True.