I had a HUCA recently I was afraid wouldn't work because of call notes. I had accidentally double-booked myself on a flight (paid for two tickets, same name same MP number), didn't notice till I went to check in. Called and first agent was adamant I had to pay a change fee (she read me times and dates I had made the two purchases, didn't care that I hadn't received a confirmation email for the first one), didn't want to let me off the call without at least cancelling the reservation, but I said I had to go and hung up. Was terrified she was either cancelling it anyway or writing up notes saying not to refund, so I called back right away and second agent was very helpful, said "whoops, looks like you're double booked; let me refund one of these for you".