This tooth has what is called an "accessory canal," which is a canal that splinters off in an unexpected direction. This tooth had to be retreated due to the first root canal failing, which is evident by the dark area, which is where the arrow is pointing. Upon retreatment, the accessory canal was found and filled, and the prognosis for this tooth is good.

This is a different tooth that also had an accessory canal, which is where the arrow is pointing. The prognosis for this tooth is also good but as with most root canals a periodic exam and x-ray is needed to detect any problems should they arise.