I'm often called paranoid, but really, it's a good thing. If you aren't paranoid, you aren't thinking hard enough. Computers require 100% correctness - if you miss one step or make one mistake the result can be catastrophic. You constantly have to think "what could go wrong", "what can I do to stop that happening", "if it still goes wrong, how can I fix it". Generally this means doing extra work up front which may never be used, however it's a wise insurance policy.

Let's have a concrete example, recently someone asked on a mailing list "how can I make my website secure". There were several good replies which answered the question - "keep up to date", "perhaps use SELinux". However no-one was sufficiently paranoid for my liking.

My answer was slightly different, it was "do those things, but assume you will still be hacked". Two things really need implementing for this - backups (tested of course!) and an IDS (Intrusion Detection System) to tell you when you have been hacked. I use Osiris as my IDS - this allows me to watch to see what files are changed - if anything unusual happens, I am notified and can take appropriate action. Furthermore I also run chkrootkit which can alert if machines have been cracked.