[patio11]

Bug free isn't a requirement for the vast majority of applications. Attaining it is very expensive, and most people/companies are not willing to pay the price.

Probably the closest the world has ever come to bug-free code of substance is that produced by NASA for their space shuttles, and it cost ~$1,000 per line. (http://history.nasa.gov/sts1/pages/computer.html) This would make the typical YC app cost tens or hundreds of millions just to launch, and would put something like e.g. the operating system for your cell phone outside of the budget of most nations.

There are a host of other engineering tradeoffs that have to be made, too. In addition to prioritizing cost over bugs, shipping speed over bugs, feature set over bugs, being able to actually hire programmers over bugs [+], etc, are all important.

[+] Many programmers would consider the type of work environment you need to get bugs down to zero to be an oppressive place to work in. Among other things, it will quite literally try to crush any creativity out of you, turning you into an automaton which exactly implements the specs given to you. Those specs were dictated by Really Important People. You're not one. Write your function. Your last function was non-compliant with rule 436 2a subsection b. Improve your performance or we'll find someone who is not a threat to our software quality.