This may not be a new topic, as I know that some transaction session handling was changed earlier this year.
I am seeing a case where a user went all the way the the registration form for an event, and then completed an on line payment. The payment confirmation was received, but no registration confirmation was received, and there is no record of the registration data in the database.
I am running DT Register v2.8.4 on Joomla 2.5.8.
The payment gateway is Authorize.net.