The article discusses a subtle bug that can arise from modernizing Go error handling, specifically when switching from type assertions to the errors.As function. It illustrates how mismatched pointer and value types in error handling can lead to silent failures that alter program behavior unexpectedly, emphasizing the importance of clarity in error type usage and documentation. The author proposes strategies to prevent these issues through compile-time assertions and consistent error practices.

Designing custom error types in Go can lead to subtle bugs if best practices are not followed, particularly regarding the implementation of the Is method. A common pitfall arises when using the errors.Is function within an Is method, which can violate Go's error handling semantics and create misleading error equivalences. Developers should prioritize direct comparisons for error equality in custom Is methods to avoid such issues.