Never say, “You’re wrong.” – If the guest thinks Shiraz and Syrah are two different grapes, don’t correct them. Emphasize that Australian Shiraz and California Syrah are certainly two different wines that each have their strong points and there are many people who prefer one over the other.
If you are wrong, admit it quickly and emphatically – Which would make a guest feel better about your service, ” yes, I was wrong about telling you ‘Xyz’. I apologize for that.” or “I could be wrong about that but I will check it so I do not make that mistake again.”