An attendee tries out an Apple Watch folowing an Apple event in San Francisco, California March 9, 2015. (REUTERS/Robert Galbraith)

It's been roughly four days since the Apple Watch first started showing up on regular people's doorsteps, and we're already drawing some lessons from its first days in the wild.

While many Apple Watch buyers are still waiting for their watches -- delays that The Wall Street Journal reported Wednesday could be due to supply issues for its "taptic" alert sensors --  some customers are already seeing problems. A handful of users have said that tattoos on their wrists seem to interfere with the Watch's ability to track their pulse. The darker that tattoo, it seems, the worse the issue is. One reddit user has also reported that black ink in tattoos also prevents some notifications from showing up on the watch, though that complaint hasn't been as common.

Apple did not respond to a request for comment.

The problem is likely caused by the fact that ink in users' skin interferes with the light technology that Apple uses to sense your pulse. The light sensor allows the watch to be loose on the wrist and still read blood pressure. This is how it works, via Apple Web site:

Blood is red because it reflects red light and absorbs green light. Apple Watch uses green LED lights paired with light‑sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment. When your heart beats, the blood flow in your wrist — and the green light absorption — is greater.

It makes sense, then, that ink in your skin could mess with the light-based technology Apple's using to read your pulse (though it is not affected by skin tone). And Apple has not been the only company to experience these problems. At least one reddit user with a Fitbit HR, which uses the same technology  to measure heart rate, has reported similar interference that could be tattoo-related.

Apple's community forums are also full of general questions about Watch functions, including some about the health sensors.

As we've said before, this is clearly a product that needs to be on the wrists of normal people for a while before anyone "gets" it. As more developers play around with the device and make more apps for it, the Watch could evolve.

There is one type of app we know won't be welcome on the watch, however. Updated guidelines for developers make it clear that Apple will not accept apps designed solely to make the Watch, well, a watch.

"Watch Apps whose primary function is telling time will be rejected," according to the company's guidelines.

Banning watch apps for a smartwatch might strike some as a little baffling. And, so far, Apple has offered no explanation.