The lesson addresses a bug related to file offsets when handling multiple files or standard input in an application. The core issue arises when the same file descriptor is used, leading to incorrect results from count functions due to the file offset being at the end of the file. The solution involves implementing a more efficient single-pass algorithm to count bytes, lines, and words by reading through the file only once, rather than iterating multiple times. Additionally, it emphasizes the importance of error handling and the limitations of unit tests in capturing all potential issues, particularly with standard input. The lesson concludes with plans to enhance coding output and further explore advanced testing and concurrency in future modules.