Apple has said some of the functions on its new smartwatch may not work properly when it is worn over tattoos.
Darker-coloured artwork and even changes in darker coloured skin types can fool the light sensors on the back of the watch.
The problem is not exclusive to the Apple Watch, which performed well in independent tests.
But it does show the manufacturer has not solved the sensor problem.
"Permanent or temporary changes to your skin, such as some tattoos, can... impact heart rate sensor performance," Apple said on a support page on its website.
"The ink, pattern, and saturation of some tattoos can block light from the sensor, making it difficult to get reliable readings."
The watch uses green LED lights combined with light-sensitive photodiode sensors to detect the amount of blood flowing through the wrist, which can then be used to calculate heart rate.
Other problems have been reported. Matt Siegel, a journalist for the Reuters news agency said that "the watch locks on tattooed skin and does not deliver the soft pings that alert a user to incoming messages".
"The heart rate readings were also significantly different on tattooed and untattooed wrists."
Videos posted to YouTube have shown users with wrist tattoos attempting to log a work out session, only for the watch to appear to intermittently pause the stopwatch when it failed to detect the wrist.
The problem is not unique to the Apple Watch.
Several smartwatches and wearable fitness devices that use similar sensor technology have also been reported to struggle when worn on darker coloured skin. In these cases the amount of light reflected back from deeper-coloured pigmentation of the skin is less than the device is calibrated for.
The technology in Apple's watch does not appear to be of a low standard. It performed well in independent tests against leading heart rate monitors, according to Consumer Reports.
The repair site ifixit.com has taken apart the Apple Watch and reported that its heart rate monitor system is more advanced than most, offering potential functions that Apple is not currently promoting.
"Apple's heart rate monitor is actually a plethysmograph," it said.
"It looks and acts like a pulse oximeter, but Apple isn't claiming it can measure your blood oxygen level. Why? Beats us. Our best guesses involve FDA [US Food and Drug Administration] regulations."
It seems that the sensor and monitor functions of the Apple Watch are fairly advanced, but that the technology giant has yet to solve the known problems presented by darker-inked skin.
"We're not surprised the Apple Watch has run into problems with tattoos as it uses similar optical heart rate monitoring tech as the Fitbit Charge HR," Sophie Charara, contributing editor of wearable technology website Wareable, told the BBC.
"Apple now needs to offer users the option to disable the pin code security when the smart watch doesn't detect your wrist.
"The winning wearable tech in the next few years will be the devices that work with our bodies, not the ones that ignore them