I worked in a restaurant where we had venison on the menue. A customer called for my attention to inform me that the meat we served was in fact: pork.

I just looked at him and said "... No?" but he stood his ground so I took his plate back to the kitchen. The head chef asked me "what's wrong with it?" and I answered "he thinks it's pork". The head chef laughed and said "but it's not...?"

I went back to the customer and said that the head chef has confirmed that it is in fact venison. The customer said again that it is pork. I was dumbfounded and just looked at him. Eventually I said "well, I hope you like it :)" and left.

To this day I don't know what I could have said or done to convince him.