This has become less of an issue with such comprehensive information sharing nowadays (some credit card issuers don't even allow travel notices anymore - apparently everyone knows where everyone's going) but sometimes still is. One time more than a decade ago my relative called me from the airport while checking in on a mileage ticket which I had paid for as the airline was wanting the Credit Card number. I would imagine these check-in desks have a system that will simply not allow moving on to the next step let alone final Boarding pass issuance until the number is provided and all security alerts are resolved... so the agents are helpless in this situation. This especially occurs when the Credit Card is from a different country and/or someone else paid for the ticket. Anyways since then I always handwrite the last 4 of the credit card, whose and how many miles were used if it's a mileage redemption, and the amount paid on a printout of nearly every reservation (unless it's domestic or straightforward), so that if I do, I have the info. And then I make sure I have every card in my wallet.