Be sure the vintage is what the guest ordered and the type of grape is what they wanted. They may have ordered a blend like a Meritage but you only found the Cabernet Sauvignon bottle of that producer or they wanted the Chardonnay but the Sauvignon Blanc wine of the same producer lept into your hand instead.
Not confirming the vintage the guest ordered is the most common service error in identifying the correct wine.
All of the “correct wine” confirmation must be completed before you open the bottle.