A few days ago I was in the kitchen with my younger son. He sneezed. I said, "Gesundheit." He sneezed again. Again, I said, "Gesundheit." To which he replied peevishly, "I covered my mouth!"
So for the last eight and a half years every time he's sneezed, and I've said, "Gesundheit," he thought I was scolding him for not covering his mouth!