Common complications of root canal treatment are:

• Root fillings
  Occasionally, especially if the root is curved or curly, the root filling fails to reach to the tip of the root. Your dentist may be only able to seal up to the point of the curvature and this results in a seal that is less than perfect. Fortunately, this rarely poses a problem as the seal above the curvature, coupled with the seal obtained from the filling could be adequate. However, if the tooth starts giving problems your dentist may refer you to a specialist.
• Fractured instrument
  The files and drills used in root canal treatment are often very fine and delicate. Although very resilient, they can sometimes fracture. If they are not easily removed, your dentist may choose to leave the fragment in place and seal around it. This will often be enough and the tooth could settle down well. However, if pain persists you may have to see a specialist or your dentist may choose to do an Apicoectomy.
• Split root
  During the seal, a great deal of pressure is used to fill the root canals. This can sometimes cause the root to split – If this happens, the root may have to be removed in a procedure called a Hemi–resection or sometimes the tooth may have to be extracted.

Fortunately, as modern root canal treatment has been around for almost a century, dentists are able to overcome most complications with very little reduction of the success rate.