Did you know some people are actually born with an extra navicular bone, called an accessory navicular bone? This foot abnormality is congenital, and usually doesn't cause a whole lot of problems.
In fact, it is easy to mistake an accessory navicular bone for an enlarged single navicular bone, but it is easy to see the difference on an X-ray.
Most of the times an accessory navicular bone won't cause any problems, but if they do occur, it's likely to be foot pain and swelling around the ankle. This can look somewhat like ankle tendonitis, so it can take a while for doctors to come up with a diagnosis of navicular bone pain.
However, it's easy to fix -- the majority of problems can be treated non-surgically, but if the navicular bone pain continues, an easy surgery can clear it up.