mozilla/kuma — pull request #74 by ubernostrum (2011-12-27 17:05:48+00:00)
Title: Bug 710529: Force latin-1 decode of HTTP User-Agent

So, like the commit message says, this is the only place that we can be getting non-Unicode but also non-ASCII-containing strings, so latin-1 decoding it should just make this traceback go away.