This has happened to me also in the past though generally it has been associated with a non completed record. This includes situations where contents of mandatory fields are completely missing from the record. When i have questioned the registrant about this I generally get comments like "I had to restart the browser" or "my computer crashed" , "I left the form open whilst i had tea" etc. It would be interesting to know how DT Register has been programmed to react to these type of events.
We're having the same problem. I thought it was the SIM plugin so I switched it back to AIM. The problem we're having is we have to now pay CLOSE attention to our emails and registrations. We recently had someone get access to a paid webcast because not only did it give her a free registration, but it assigned her to the group. It's one thing for someone to not get charged for a conference (we can go tell them we never got paid and ask them to check their credit card)...but to have it give someone access to paid content is frustrating. So far no answer to my tickets going back weeks.