Beginning a few days ago, the mongo shell cannot authenticate to mongos during the C Driver's Evergreen startup. For an example task failure, see https://evergreen.mongodb.com/task/mongo_c_driver_windows_64_vs2010_integration_test_latest_sharded_7e0105c26ef1b295739465afc8d4dac6218f0ea5_16_07_14_17_31_13.
It logs:
[2016/07/13 22:01:21.860] connecting to: 127.0.0.1:27017/admin [2016/07/13 22:01:21.862] MongoDB server version: 3.3.9-222-g9e7a989 [2016/07/13 22:01:21.873] 2016-07-14T02:01:21.873+0000 E QUERY [thread1] Error: Authentication failed. : [2016/07/13 22:01:21.873] DB.prototype._authOrThrow@src/mongo/shell/db.js:1414:20 [2016/07/13 22:01:21.873] @(auth):6:1 [2016/07/13 22:01:21.873] @(auth):1:2 [2016/07/13 22:01:21.878] exception: login failed