Are there any unlisted badges?
I almost envisioned easter egg badges on embarassing activity: Like "First time you tried to upvote your own question" or "First question that recieved more upvotes from comments than from the question"
There is the Hacker Badge which has never been awarded.
In the linked blog post, Jeff also says
(There might even be some other secret badges out there.. who knows?)
but if there are others they've remained secret.