Ask the customer for any type of receipt of sale, check our computer history, and ask the customer if they remember who they placed the order with. If none of that helped, then I would apologize for the mishap and assure the customer that I will personally fill the order right here and now.
It was the basic questions anyone would answer for a sales position. Why I would be good at sales, what about sales do I find most challenging, etc... The most difficult or surprising thing that I recall was this: I was given an ink pen and asked to sell this pen to her.
I treated the pen as a cell phone since my last job was in wireless sales. I treated it as if she had just waled in to the pen store, I greeted her and asked the common questions that you would ask to discover what type of pen she needed. Once I knew exactly what she would be needing the pen for I directed her to our best sellers and continued until we found a pen that was a good fit for her.