while profile level is 2, auth using a user not exists in the db will lost connection. and this problem still exists in current 2.1.0 version.

$ ./mongo
MongoDB shell version: 2.0.2
connecting to: test
> use admin
switched to db admin
> db.auth('admin','.....')
1
> use test2
switched to db test2
> db.getProfiling
db.getProfilingLevel( db.getProfilingStatus(
> db.getProfilingLevel()
0
> db.setProfilingLevel(2)

{ "was" : 0, "slowms" : 100, "ok" : 1 }

>
bye

$ ./mongo
MongoDB shell version: 2.0.2
connecting to: test
> use test2

switched to db test2
> db.auth('aa','aa')

Tue Feb 28 15:34:15 DBClientCursor::init call() failed
Tue Feb 28 15:34:15 query failed : test2.$cmd

{ authenticate: 1.0, user: "aa", nonce: "b813a5f7a1f96482", key: "f8535382b1996acf257ddbbae7f4f638" }

to: 127.0.0.1
Tue Feb 28 15:34:15 Error: error doing query: failed shell/
collection.js:151
Tue Feb 28 15:34:15 trying reconnect to 127.0.0.1
Tue Feb 28 15:34:15 reconnect 127.0.0.1 ok