“A growing number of users have turned to social networks claiming that full sleeve tattoos, ones that stretch to cover the wrist, interfere with the Apple Watch’s heart rate sensor and skin contact registration. Consequently, some Apple Watch users with tattoos have been unable to receive notifications and are frequently prompted to re-enter their passcode, if one is set, on the wrist-worn device.”

Dark tattoos can throw off Apple Watch’s heart rate sensor
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Some Tattoos Found to Interfere With Apple Watch’s Sensors – Mac Rumors
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The Apple Watch, like your grandmother, has a problem with your tattoos. – The Washington Post