Odd issue with using flight credit today. I canceled a roundtrip flight, used the flight credit to rebook the same flights on different dates at a lower price. The transaction did not go through with a message about payment not authorized. Both the flight credit and old flights were gone (because they were canceled), and the new flights did not appear in my itineraries. I called reservations and was told that the UA system showed the canceled flights were sent to their "refund desk" for a refund, even though there was no mention of a refund when I had tried to rebook the new flights. After a bit of back and forth on the phone (during which time the new flights I had booked appeared unticketed in my reservations), the agent was able to restore the credit and issue the tickets as well as the FFC for the lower price. I canceled and rebooked many times, but never encountered this problem. Why would a ticket be sent for a refund after a FFC had already been issued for its value?