http://docs.mongodb.org/manual/reference/limits/ does not clearly document that $ can't be in a database name.

For example:

> use $dsf
switched to db $dsf
> db.col.insert({})
WriteResult({
"nInserted" : 0,
"writeError" :

{ "code" : 17320, "errmsg" : "cannot do createCollection on namespace with a $ in it: $dsf.col" }

})