With today's $36 Ding fares, I cancelled and tried to rebook a few higher-priced tickets that I had made earlier.

I do this all the time, but here's the issue I ran into today -- simplified for illustration purposes. This also happened to me a couple months back when they had those $25 fares. Could be due to system overload...

1. Cancelled a one-way $100 reservation, getting credit in the form of a confirmation numbers (PNR, I think they call it).
2. Used the credit to book a lower-priced $75 Ding ticket, leaving $25 in that confirmation number.
3. While making a 2nd $75 reservation, i wanted to use the remaining $25 as payment. Suprisingly, it said I still had $100 available.

I was kind of stuck at that point. It "took" the entire $100, even though I had used $75 of it in step (2). It did not give me any option for entering credit card info, since the system thought no additional funds were owed. I did not want to lose the Ding fare, and also I was a little bit curious, so I clicked "Purchase" to see what would happen. Surprisingly, it let me purchase the ticket.

After I get the confirmation email, I will sort it out with Southwest -- I'm not trying scam anyone here, after all -- but this seems like a flaw in the systemm, that bookings using ticketless funds are not processed immediately, and that the funds are available for "double dipping".