What is the primary concern with a developer testing their own code?

The primary concern with a developer testing their own code is that developers are not objective about their own work. This lack of objectivity stems from their emotional and cognitive investment in the code they have written. When a developer creates a piece of software, they may have biases that affect their ability to identify flaws or issues. They might unconsciously overlook errors because they know the logic behind their implementation, or they might interpret ambiguous results in a way that favors the correctness of their code.

In testing, objectivity is crucial for effectively identifying defects and ensuring the software meets its requirements. Independent testers, who have not been involved in the development of the code, typically have a fresh perspective, enabling them to highlight potential issues that the original developer might miss. This creates an essential layer of quality assurance, as independent testers are more likely to challenge assumptions and catch errors that developers may inadvertently justify or rationalize away.