3-server replica set (primary, slave, arbiter). Primary steps down, the next query (actually any queries) fail with NullReferenceException with the following stack trace:
MongoDB.Driver.DLL!MongoDB.Driver.MongoServerInstance.AcquireConnection(MongoDB.Driver.MongoDatabase database) Line 183 + 0x8 bytes
MongoDB.Driver.DLL!MongoDB.Driver.MongoServer.AcquireConnection(MongoDB.Driver.MongoDatabase database, bool slaveOk) Line 893 + 0xf bytes
MongoDB.Driver.DLL!MongoDB.Driver.MongoCursorEnumerator<System.__Canon>.AcquireConnection() Line 184 + 0x42 bytes
MongoDB.Driver.DLL!MongoDB.Driver.MongoCursorEnumerator<MongoDB.Bson.BsonDocument>.GetFirst() Line 194 + 0xc bytes
MongoDB.Driver.DLL!MongoDB.Driver.MongoCursorEnumerator<System.__Canon>.MoveNext() Line 126 + 0x8 bytes

It appears that connectionPool is null, probably because MongoServerInstance.Disconnect due to the connection error. Might be related to CSHARP-217 and CSHARP-233. I'll post more information later.