Today, I got a strange exception:
System.NullReferenceException: Object reference not set to an instance of an object.
at MongoDB.Driver.MongoServer.AcquireConnection(MongoDatabase database, MongoServerInstance serverInstance) in mongodb-mongo-csharp-driver-1091cad\Driver\Core\MongoServer.cs:line 826
at MongoDB.Driver.MongoServer.AcquireConnection(MongoDatabase database, Boolean slaveOk) in mongodb-mongo-csharp-driver-1091cad\Driver\Core\MongoServer.cs:line 819
at MongoDB.Driver.Internal.MongoCursorEnumerator`1.AcquireConnection() in mongodb-mongo-csharp-driver-1091cad\Driver\Internal\MongoCursorEnumerator.cs:line 137
at MongoDB.Driver.Internal.MongoCursorEnumerator`1.GetFirst() in mongodb-mongo-csharp-driver-1091cad\Driver\Internal\MongoCursorEnumerator.cs:line 147
at MongoDB.Driver.Internal.MongoCursorEnumerator`1.MoveNext() in mongodb-mongo-csharp-driver-1091cad\Driver\Internal\MongoCursorEnumerator.cs:line 83
at MongoDB.Driver.MongoDatabase.GetCollectionNames() in mongodb-mongo-csharp-driver-1091cad\Driver\Core\MongoDatabase.cs:line 560
at MongoDB.Driver.MongoDatabase.CollectionExists(String collectionName) in mongodb-mongo-csharp-driver-1091cad\Driver\Core\MongoDatabase.cs:line 277

My code:
var connectionString = "mongodb://server1,server2/databasename?replicaSet=replicaSetName"
var urlBuilder = new MongoUrlBuilder(connectionString);
urlBuilder.SafeMode = SafeMode.True;
var db = MongoDatabase.Create(urlBuilder.ToMongoUrl());
db.CollectionExists("collectionName"); // Exception was thrown here

The replica set consists of three machines. At the time of failure their roles were as follows:
server1: primary
server2: secondary
server3: arbiter (it was not specified in the connection string)

The exception only appeared once (my code is called from a lazy initialization code that is from a web page, so when I refreshed the page, the code that failed got called another one time and this time it was OK). This is a first operation on the database, slaveOk is set to false, so it appears that there's a problem in ReplicaSetConnector.Connect() so that it returns before it's able to acquire connection to primary, or instance list or metadata becomes damaged so that MongoServer.Primary returns null even if there is an instance, to me this looks like a race condition.