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Demand for the Apple Watch has clearly outstripped supply during the initial launch period. The small number of wearables trickling out to end-users can be blamed on trouble with the taptic engine, something Apple discovered prior to launch.

Apple also didn't notice at least one issue plaguing the Watch before it reached consumers: Incompatibility with ink.

Apple's quality-testing procedures helped it catch a flaw in the taptic engine before the device shipped, reports the Wall Street Journal. Apple tasked two companies to make the taptic engine, AAC Technologies in China and Nidec in Japan. The taptic engine uses a motor and sliding rod to create gentle taps on the wrist to notify wearers of incoming calls and messages.

Apple's quality assurance metrics found some taptic engines from AAC Technologies were bound to break over time. This forced the Watch maker to cut AAC from its supply chain and shift all taptic engine sourcing to Nidec.

Consumers needn't fear their Watch is defective, according to Re/Code. "I believe no faulty Apple Watches were shipped to consumers," said Patrick Moorhead, founder of Moor Insights & Strategy, in a statement to Re/Code. "I don’t think this is damaging at all."

Losing one of the two suppliers seriously curtailed Apple's ability to manufacture the smartwatch at volume, which has contributed to the limited supply of the device. People placing orders for the Watch on Apple's website today may not see the device ship until June or later, depending on the model.

"Our team is working to fill orders as quickly as possible based on available supply and the order in which they were received," Apple said. "We know many customers are still facing long lead times and we appreciate their patience."

Apple's quality assurance measures apparently missed the boat on tattoos. Apple Watch owners with dark tattoos on their wrists reported problems with the wearable. iMore investigated the issue and agreed there is in fact a problem.

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"Apple uses various spectrums of light to track the blood flow through your skin. Anything that reduces that light's reflectiveness -- ink pigmentation within your skin, for example -- can interfere with that sensor," explained iMore. "Natural skin pigmentation doesn't block light the same way artificial ink pigment or even scar tissue does, so you shouldn't run into a problem if your skin is naturally darker."

iMore's tests showed black and red tattoos were most likely to cause problems, while lighter colors like yellow were less likely to cause problems.

Apple hasn't yet responded to this issue.