The “O0PS” moment whilst doing endodontic treatment. Difficulties encountered during endodontic treatment can be a cause of frustration for both clinician and patient. Complications may impact the outcome of the endodontic treatment. Clinicians should understand how each of these complications can occur, be prevented and predictably rectified. It is also important for clinicians to understand the impact of a specific complication on the treatment prognosis and to appreciate how this may differ in each case. There may be scenarios where a complication does not significantly affect the prognosis, whereas in other situations, the same complication will be detrimental to the outcome of treatment. An appreciation of the clinical factors which determine prognosis is important, so that the correct intervention is chosen, and the patient is correctly informed of the likely sequelae. Topics to be covered include: Incomplete analgesia Perforation – pulp chamber - root Instrumentation separation Unexpected anatomy Ledging Calcification in the root canal system Hypoclorite injuries Who to contact when things go wrong Obturation errors