☛ More than the normally anticipated number of root canals in a tooth (leaving one of them uncleaned)

☛ An undetected crack in the root of a tooth

☛ A defective or inadequate dental restoration that has allowed bacteria to get past the restoration into the inner aspects of the tooth and recontaminate the area

☛ A breakdown of the inner sealing material over time, allowing bacteria to recontaminate the inner aspects of the tooth