On a live site, I don't have many opportunities to try to reproduce the problem, unfortunately.
There were no errors *reported*, either by the customer or by Authorize.net. As far as I know, the site was not down.
Here's what I know -
The failed transaction was completed on 3/3/09 at 14:21:02, and was approved by Auth.net. I have all of the details that were sent to Auth.net saved in a logfile that is written after each transaction has been submitted, so that I can see exactly what has been set to and returned by Auth.net. Less than an hour later another transaction was completed successfully. If you'd like to see this logfile data, I'll be happy to provide it.