What can lead to failure in the instrumentation of an anterior root canal if straight line access is not achieved, but is the exception?

Achieving straight line access is crucial in endodontics, particularly for anterior root canals, as it allows for proper instrumentation and cleaning of the canal system. When considering the exception among the options provided, creating a lingual shoulder in the canal does not typically lead to failure in instrumentation in the same manner as the other issues do.

While creating a ledge, zipping at the apical foramen, or instrument separation due to deflection directly impede the smooth progression of instruments down the canal and compromise the ability to reach the apical area effectively, the creation of a lingual shoulder may complicate access but does not necessarily result in immediate failure of instrumentation. It may make navigation more difficult but does not completely prevent access as the other issues do.

In scenarios where straight line access is compromised, the endodontist may still manage to negotiate the canal despite these challenges. Therefore, recognizing that the lingual shoulder can indeed create difficulty but it is not as directly detrimental to success as the other potential complications makes it the exception in this context.