Trying to avoid shipping bugs is a defense-in-depth affair, because fundamentally tests can only show the existence of a bug, but not that a program is bug-free

Trying to avoid shipping bugs is a defense-in-depth affair, because fundamentally tests can only show the existence of a bug, but not that a program is bug-free. It starts right in the editor, where syntax highlighting can help with spotting mistakes such as writing 010 instead of 10 (0 prefix for octal is a terrible idea). Pretty sure that bug-free code is one of the prides of programmers worth their salt; but there's no helping, when you forget about something while coding, it is very likely that you also forget to test it. That's why we need a separate tester.