in logging/__init__.py line 328 there is a simple line:

     msg = msg % self.args

if that line fails this failure will be logged, not the line itself. i suggest to change the line to something like:

    try: msg = msg % self.args
    except TypeError: msg = 'logging formatting error with "%s" %% %s' % (
        msg, self.args, )

to be able to pinpoint the actual wrong formatting string (the stack trace doesnt help here).