You must be methodical and systematic in order to find faults. You must also be methodical and systematic in your management and handling of them. Before releasing code (or checking it into source control), you are the only person who'd be bitten by its gremlins. But as soon as it leaves your care, code takes on a life of its own. It's no longer just you who is concerned with its faults. The rules change as more players join the game:
A programmer will find problems at the codeface—in his own code and in other people's.
The code integrator will find errors as components are glued together.
The QA department will find faults in the product as it tests.
With so many people finding so many problems while others are simultaneously trying ...