Q:

Twilio 11200 errors in sms status callback

We are using node.js to send SMS to customers, and recently added the status callback to log sent and delivered statuses.  We are also now seeing a ton of 11200 errors in our Twilio logs on calls that apparently were successful.  Looking at the debugger, I see this in the response body:

Twilio was unable to fetch content from:
  https://api.mycompany.com:443/api/1.0/sms/response Error: Error
  reading response: Response does not contain content type

and the message text has:

Msg   "Request to StatusCallback URL was unsuccessful."
  httpResponse  "502"

On our side, the handler for /api/1.0/sms/response takes the MessageSid sent with the response, matches it to one in the database, then looks at the MessageStatus.  For "sent" it logs the current time in a sent field and saves it back to the database, likewise for delivered.  At the end, it just does a res.end() as shown in the node.js example.
Here's the code:
app.post('/api/1.0/sms/response', function(req, res) {
    var post_id = req.body.MessageSid || "";
    var status = req.body.MessageStatus || "";
    var item = Item.create(globals);

    if (post_id === "") {
        globals.logger.error(moduleName, "POST sms/response", "Missing MessageSid: " + JSON.stringify(req.body), true);
        res.end();
        return;
    }

    item.loadFromPostId(post_id).then(function() {
        if (status.toLowerCase() === "sent") {
            item.sent = new Date();
        }
        else if (status.toLowerCase() === "delivered") {
            item.delivered = new Date();
        }

        item.save().then(function() {
           res.end();
        });

    });
});

Looking at a couple of the errors, they are when Twilio is trying to send a "delivered" status to us.  Checking our database for the MessageSids from these errors, the matching item has a timestamp in the "delivered" column, so it made it all the way through the code to the save.  
We also have nothing but 200 responses in our nginx log (which we are using in front of node) for this endpoint, no upstream timeouts and no 502s.  So I don't know where this error is coming from, or how to get rid of it.
And yes, we should be handling undelivered, etc., but this was just a quick pass at getting some responses in.

A:

You should respond with HTTP status code 204:
res.status(204).end();

The 204 status is still a success, like 200, but specifically means you are not sending any content back (https://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html).
Note that the following also works, and doesn't need the end():
res.sendStatus(204);

