It's not a Ruby gotcha, it's a Rails routing gotcha. You can specify alternative formats on any path, e.g. you can access /path to get HTML (or whatever the "de

It's not a Ruby gotcha, it's a Rails routing gotcha. You can specify alternative formats on any path, e.g. you can access /path to get HTML (or whatever the "default" format for that controller method is) or you can access /path.json or /path.xml and if the controller method specifies handlers for those formats, you get that format. So if you allow a username like "john.doe" and the route is something like /john.doe then Rails will interpret the "john" as the ID part of the path and "doe" as the format part of the path. You can override this in your routes to support periods but then you do lose the capability of accessing alternative formats which sometimes can be useful.