Handling concurrency in applications can lead to non-deterministic behavior, particularly affecting the consistency of output when processing multiple files. This lesson emphasizes the importance of achieving deterministic printing in the standard output to enhance user experience. By modifying the testing strategy to ensure that the expected output corresponds directly to the order of input files, a clearer approach using multi-line strings simplifies the process. Additionally, collecting results in a slice that preserves the input order and indexing them correctly are crucial for maintaining predictable behavior in tests. This structured approach not only resolves flaky tests but also sets a solid groundwork for future features, such as testing CLI flag behavior in upcoming lessons.