One problem has caused much irritation to users, because Apple Watch requires a PIN security code to detect once it is set in the wrist of another user. Heart rate monitor works by illuminating the skin with infrared color or green, but tattoos can interfere and hinder the functioning of the clock.
"The changes permanent or temporary to your skin, like tattoos, for example, can affect the functioning of the heart rate sensor," writes Apple. "The color and tattoo model block the light from the sensor, making it hard to function."
