I'm experiencing a rare issue where existing customers completing checkout place the order successfully. However, the following strange behavior occurs:

• The order confirmation email is not sent,
• The order is assigned to the customer correctly, yet customer_id is set to 0, and
• If the customer places the order again, it works correctly (order confirmation email sent and customer ID assigned correctly) however, the increment_id assigned is precicely the same as the first order and as a result there is a duplicate order increment id, with both orders active.

As I mentioned, the issue is quite rare, we receive over 20 to 30 orders a day, and it only seems to happen once a day, if that.

Edit: as per comment, it is useful to note that I am using an extension which allows alphanumeric order increment_ids, Aschroder Set Custom Order Number, our orders follow a trend as follows: OR1000, OR1001, OR1002, OR1003, etc. We have been using this extension for over 10,000 overs without this issue appearing before.

I am also using ASchroder.com SMTP Pro Email - Free and Easy Magento Emailing for SMTP, Gmail or Google Apps email to integrate google mail support for Magento.

Update 1: Looking at exception.log the following entry is notable which occured around the time of the incidents:

 'Zend_Mail_Protocol_Exception' with message 'Unable to connect via TLS' in /html/lib/Zend/Mail/Protocol/Smtp.php:211

It seems like this could cause the order creation process not to reserve the order increment id on the quote model.

Is there a way that I can diagnose what is causing this behavior? All other orders placed seem functionally correct.