Initialize an unconnected MongoClient with the wrong password, then do an operation that triggers it to connect and authenticate:

c = MongoClient(
    "mongodb://user:wrong@%s:%d/pymongo_test" % (host, port),
    _connect=False)

c.pymongo_test.test.find_one()

Expected: OperationFailure. Actual: AutoReconnect.

MongoReplicaSetClient handles this case correctly.

I revealed this bug in the process of fixing PYTHON-516. The fix is moderate-risk, low-reward, so I'm putting it off until PyMongo 2.6.