This one popped into my head as I was fixing my lunch, and it put me off my sandwich.
I was babysitting my little cousin, and I was giving her her lunch in her high chair while my uncle, home for lunch, was making himself a peanut butter sandwich. He dropped the slice of bread he was smearing peanut butter on, and it landed peanut butter side down (of course) on their kitchen floor. They don't keep the cleanest house, and there was visible dirt and pet hair on the spot where the bread landed, so it never entered my mind that he would do what he did. He picked up the piece of bread (and I could see things sticking to the peanut butter), and instead of throwing it away and starting over, he finished making his sandwich and took a bite out of it. When I expressed disgust, he shrugged and said, "A little dirt never hurt anyone."
It's not just germs; it was visible dirt! I just could not get past that.