There's always one. You should remember this: there is ALWAYS, at least, one flaw. Just one hole through which a cracker could get into your system and do things you would not do to your worst enemy's laptop. Talking seriously, the Golden Rule of Security is: "Security is just a state of mind". That means that you THINK you are secured. When you come to believe such a thing, you automatically stop worrying. And that is a Bad Thing (tm).
This is, I am sad to say, what a lot of system administrators do. And that is something attackers tend to take advantage of. The first thing a cracker will do is among one of these: set up sniffers, read eMail and crontab files, then try to discover the "when's and how's" of the system administrator, etc. They, of course, try to hide their tracks. But most of them just go and install a Rootkit.
Download the paper in TXT format here.