Psychological Considerations in Debugging
Debugging is as intellectually demanding as any other software-development activity. Your ego tells you that your code is good and doesn't have a defect even when you've seen that it has one. You have to think precisely—forming hypotheses, collecting data, analyzing hypotheses, and methodically rejecting them—with a formality that's unnatural to many people. If you're both building code and debugging it, you have to switch quickly between the fluid, creative thinking that goes with design and the rigidly critical thinking that goes with debugging. As you read your code, you have to battle the code's familiarity and guard against seeing what you expect to see.
For an excellent discussion of ...