I stick my finger in my cat's ear and it is all over. He's shoving it so hard and far down his canal it feels like I'm scratching his brain directly. That and smooshing a brush full of pins into his face are 2 things I can never understand about him.
I only scratch the top of their head and pat their backs but never under or near mouth or paws. My cat has betrayed their trust with me too many times to earn that privilege.