I just recently saw the Oracle badge awarded. What other hidden badges are there in the system?
Is it simply one badge per tag or are there other hidden badges out there?
I suppose then that you haven't seen the tag-badges eh?
It's probably more impressive on Stack Overflow:
So the idea is that the system has badges for being a good citizen (upvotes, edits, activity) and then there's another set of badges for being an expert in your field (on SO they are language oriented, for instance) and on here they're going to be platform and the like. Other than that there really aren't any "hidden" ones. What you see is what there is.