From Network Magazine:
Ken Thompson, a designer of the Unix OS, explained his magic password, a password that once allowed him to log in as any user on any Unix system, during his award acceptance speech at the Association for Computing Machinery (ACM) meeting in 1984. Thompson had included a backdoor in the password checking function that gets included in the login program. The backdoor would get installed in new versions of the Unix system because the compiler had Trojan Horse code that propagated the backdoor code to new versions of the compiler. Thompson’s magic password is the best known, and most complex in distribution, backdoor code.