There is no such thing as a bug free software. Today I stumbled on this:
http://googleresearch.blogspot.com/2006/06/extra-extra-read-all-about-it-nearly.html
This is an unfortunate and sad truth about programming: regardless of our efforts, software will have bugs; it will crash, it will burn, it will fail. At the same time there is a hope:
http://alloy.mit.edu/
We desperately need code verification tools like this.