Are you able to reproduce this error at all? If it is just an intermittent problem then it may be difficult to determine what the cause is. If you can reproduce the error with some consistency then it will be easier to determine what the problem may be. In the failed orders that you have had to manually process was there anything common about them, like were the items ordered the same or was there a particular option that was present on these orders? If this error is occurring after the user is confirming their purchase you should be able to go into your orders table in the db to see any partial or incomplete orders, this may help to determine what the problem is. Please post back with any details that you can so we can help you look into this further.