If a root canal is done before a tooth abscesses, the success rate is around 95-97%.  Once a tooth has become infected the success rate of a perfectly done root canal drops to around 80%.
As you can see root canal therapy has an excellent track record, but it can fail.  In this case, root canals can be retreated.  The sealer material placed during the original root canal is removed, chamber is cleaned and disinfected and is re-sealed.