Throwing a fault inside a behavior and returning it to client

I'm writing some services in WCF to be called by a Silverlight client. I change status code to 200 every time a fault is to be returned, via a IDispatchMessageInspector.

It works almost perfect, but sometimes it keeps returning error 500: NotFound.

I have just written another IDispatchMessageInspector to commit changes in a ObjectContext. But when this fails, the error handler is not called.

I think by the time UnitOfWorkMessageInspector runs, the message was already been set up as a non-fault-response. How can I do both things work?

    public class UnitOfWorkMessageInspector : IDispatchMessageInspector
    {
        public void BeforeSendReply(ref Message reply, object correlationState)
        {
            if (!reply.IsFault)
            {
                try
                {
                    UnitOfWorkBase.Commit();
                }
                catch (OptimisticConcurrencyException)
                {
                    throw new FaultException("It was changed by another user. Try again.");
                }
            }
        }

I managed to make it work. I figured out my IDispatchMessageInspector for switching from status code 500 to 200 wouldn't work because the message was already built, then I replaced the message.

    public class UnitOfWorkMessageInspector : IDispatchMessageInspector
    {
        public void BeforeSendReply(ref Message reply, object correlationState)
        {
            if (!reply.IsFault)
            {
                try
                {
                    UnitOfWorkBase.Commit();
                }
                catch (OptimisticConcurrencyException)
                {
                    FaultException fe = new FaultException("It was changed by another user. Try again.");
                    reply = Message.CreateMessage(reply.Version, fe.CreateMessageFault(), fe.Action);
                }
            }
        }