Poking around the ruby code, it looks like it's any place that GitHub attempts to generate a link back to itself. The formatter generates the URI and then valid

Poking around the ruby code, it looks like it's any place that GitHub attempts to generate a link back to itself. The formatter generates the URI and then validates it against the route matcher, notices that the username constraint isn't matched and bombs out at that point. I'm not familiar enough with this code to know the potential side effects of changing that, so I'll file an issue for now, but that' the gist of it, for those curious!