The Apple Watch’s wrist detection and pulse measurement features often fail to work when confronted by dark tattoos.

300+ Reddit comments:

http://reddit.com/r/apple/comments/344b3o/anyone_with_tattoos_and_the_watch_please_read/

Quartz also reported black people and people with tattoos were having problems with Apple Watch determining they are alive, but they later rewrote that headline:

http://qz.com/394694/people-with-dark-skin-and-tattoos-report-the-apple-watch-is-having-trouble-determining-they-are-alive/