This patient came in for treatment of #11. Single root, single canal, no crown, you would think that this would be an easy root canal. However, notice the calcification of the canal. Sometimes when the crown is gone, it is difficult to determine the long axis of the tooth. You can see the the original access is getting slightly off centered toward the distal & lingual. This wise dentist knew to stop before a perforation occured.
If you look closely (select the image to enlarge), you can see the difference in color between the primary and secondary dentin. You can actually see where the canal used to be, before it calcified in. Right in the center of that secondary dentin is a small white speck. This is where the dentinal chips have accumulated in the canal. That little white spot is the remnant of the canal.