Mongo does not allow the creation of collections with an empty string as the name, but it is possible to rename an existing collection to "".

To reproduce:

> db[""].save(

{x:2}

) // it is not possible to create a new collection with an empty string as the name
invalid ns: test.
> db.getCollection("").save(

{x:2}

)
invalid ns: test.
> db.test.save(

{x:2}

) // it is possible to rename an existing collection to an empty string
> db.test.renameCollection("")

{ "ok" : 1 }

> db[""].find()

{ "_id" : ObjectId("4ede683f5833a9b2c91c8dc0"), "x" : 2 }

>