1. You go blank
This usually occurs when people are extremely nervous. If it happens, you will need to take a couple of deep breaths (which will calm you down by getting oxygen to your brain) and try to get your perspective again.
You can stall for time saying something like: "I couldnąt quite hear you exactly could you repeat that?" If you need to, you could come clean and say "I'm sorry: my mind has gone blank."
You could then explain that you are prone to nerves in interviews and your disarming honesty may save the day unless you are applying for a job as a firefighter, croupier or something else requiring nerves of steel.