"Oh you gotta be kidding me how the hell am I supposed to know that?"
The IVR had prompted the caller for the last four digits of the credit card number that was used to set up and pay for a service. The service may have been set up as long as a year previously, and many customers have more than one credit card. The question of how the hell the customer was supposed to know that had at least occurred to the IVR designer of this application. There were extra steps placed in the IVR to mitigate this scenario, but none were very effective. Lesson learned: ask for things that you know are easily available to the caller, or prepare them in advance for unusual requests that take time to track down.