eCart Failure with Authorize.net

Recently, my client received three orders through his website. Two of those three were processed normally:

1. A confirmation email was sent
2. A confirmation was received from Authorize.net
3. The order was entered into the database
4. At the end of the day, the daily summary from Authorize.net showed the two orders

The third order was not processed properly:

1. No confirmation email was sent
2. A confirmation was received from Authorize.net
3. No entry was made in the database
4. At the end of the day, the daily summary from Authorize.net showed this order
5. The merchant account reflects the proper credit

The site was built with the latest distribution of eCart4 (PHP/MySQL).

What could cause something like this to happen?