I somehow have a database named U+FEFF (i.e., the Unicode byte order mark).

$ mongo --username admin --password XXXXXXX admin
MongoDB shell version: 1.8.1
connecting to: admin
billingreplicaset:PRIMARY> show dbs
admin 0.203125GB
billing 9.94921875GB
local 1.203125GB
test (empty)
(empty

I actually had to delete the (invisible) BOM from this paste in order to submit the issue.

Needless to say I can't delete it, because the shell thinks it's an empty string (or a bad character). I have no idea how it got there. (lol)

By the way, this appeared after I set up a replica set.