What is the exception that does NOT lead to failure in the instrumentation of an anterior root canal when straight line access is not achieved?

The situation described involves the challenges encountered during root canal treatment, particularly in achieving straight-line access. When the canal's anatomy complicates access, it can lead to various complications that affect successful instrumentation.

Creating a lingual shoulder refers to the formation of a slight ledge or shoulder on the lingual wall of an anterior tooth canal, which may occur when the instrumentation is not properly directed or the canal anatomy is not adequately assessed. However, the key point about the lingual shoulder is that it does not inherently compromise the overall canal shape in a way that prevents instrumentation. It may present challenges, but it does not usually result in instrument separation, ledge formation, or zipping, which are critical complications that significantly hinder effective canal cleaning and shaping.

In contrast, separation of an instrument due to deflection, creation of a ledge, and zipping at the apical foramen are all issues that directly compromise the integrity and navigability of the canal, leading to treatment failure. Therefore, creating a lingual shoulder does not lead to the same level of failure as the other complications listed, making it the exception in this scenario.