To succinctly explain what it takes to program for the Web, here’s Minh’s Corollary to Postel’s Law:
Garbage in, normalized garbage out.
That is to say, something sensible (not necessarily an error) has to happen no matter what the program is faced with. Nothing new, but it highlights the long way we’ve come from the expectation that computers just blindly do what the operator tells it to. It makes computer science interesting, to say the least.
Even a Web application as simple as a guestbook has to take into account factors like exotic Unicode characters, browsers that perhaps pride themselves on being incompatible, localizing for any number of languages, escaping your input religiously, and excessively long names, not to mention the quagmire that spam prevention has become. If an oven had to operate by the same principle, you could bake a respectable cake no matter how many shoes you added to the mix.