An example of a WCF service that causes metadata publishing to fail such that "Add Service Reference" reports a cryptic failure.

bad-metadata.cs

[ServiceContract]
interface IChatClient
{
    [OperationContract(IsOneWay = true)]
    void Message(string message);
}

[ServiceContract(CallbackContract = typeof(IChatClient))]
interface IChatService
{
    [OperationContract(IsOneWay = true)]
    void Message(string message);
}

class ChatService : IChatService
{
    public void Message(string message)
    {
        // Echo the message
        var client = OperationContext.Current.GetCallbackChannel<IChatClient>();
        client.Message(message);
    }
}

class Program
{
    static void Main(string[] args)
    {
        ServiceHost host = new ServiceHost(typeof(ChatService));
        host.Description.Behaviors.Add(new ServiceMetadataBehavior { HttpGetEnabled = true, HttpGetUrl = new Uri("http://localhost:9091/mex") });

        host.AddServiceEndpoint(typeof(IChatService), new NetTcpBinding(SecurityMode.None), "net.tcp://localhost:9090/chat");
        host.AddServiceEndpoint(typeof(IMetadataExchange), MetadataExchangeBindings.CreateMexHttpBinding(), "http://localhost:9091/mex");

        host.Open();

        Console.WriteLine("Host is listening.");
        foreach (var endpoint in host.Description.Endpoints)
            Console.WriteLine("\t{0} {1}", endpoint.Address, endpoint.Contract.Name);

        Console.ReadLine();
        host.Close();
    }
}