To back up Dotty: it has happened many times to me that I ate at a restaurant, one person at the table ordered dessert, and the waiter assumed we were sharing it and made remarks to that effect, putting down lots of spoons.
How it irks me (but it doesn't change my tip): for the last two years I have been trying to eat healthily (I have lost 50 lbs), and I let my children order a dessert. I would dearly love to eat a dessert, but I am denying myself the dessert for health reasons, but not denying the children that joy. It makes it harder for me to maintain my discipline when the waiter puts the dessert smack in the middle of the table with lots of spoons, rather than in front of my child with one spoon. I immediately push the dessert away from myself, but still, I'd rather not have to do that.