Switching it on. The adage usually quoted runs along these lines:
``The only system which is truly secure is one which is switched off and unplugged, locked in a titanium lined safe, buried in a concrete bunker, and is surrounded by nerve gas and very highly paid armed guards. Even then, I wouldn't stake my life on it.''
(The original version of this is attributed to Gene Spafford)
A system is only as secure as the people who can get at it. It can be ``totally'' secure without any protection at all, so long as its continued good operation is important to everyone who can get at it, assuming all those people are responsible, and regular backups are made in case of hardware problems. Many laboratory PCs quite merrily tick away the hours like this.
The problems arise when a need (such as confidentiality) has to be fulfilled. Once you start putting the locks on a system, it is fairly likely that you will never stop.
Security holes manifest themselves in (broadly) four ways:
[From the FAQs of comp.security.unix]