I'm a US citizen with a British accent. In all my years living in the States I can recall two occasions in which a server chose to pad my bill with an 18% tip before giving it to me. Regardless of the quality of service received, both walked away with a zero tip and a clear explanation as to why this was the case. I normally tip like any other American would, but regard this particular practice as dishonest 'sleight of hand' at best, and attempted theft at worst - and I will not tolerate it under any circumstances.