It turns out that if you create a goal with the slug archived you won’t have any way of accessing the goal page: visiting /<username>/archived takes you to your gallery of archived goals, of course.
This is very inconvenient, not the least because it makes it a bit tricky to archive or delete such a goal; the buttons which do so being on the inaccessible goal page.
You can delete the goal, with a bit of cleverness, and a bit of curl: there’s nothing actually preventing you from manually sending that DELETE request which the deletion form on the goal page would send, if you had access to it.
Nevertheless, this is a rather painful trap to fall into. Ideally, Beeminder would prevent you from using archived as a goal’s slug at goal-creation time.