In the process of discussing error checking in the comments section of a recent post, I flippantly remarked that "all users are idiots." This is a sentiment that I expect all veteran programmers will have encountered or stated themselves on some occasion.
From a new programmer struggling with this problem, I hear this: "This would be a heck of a lot easier if users just weren't idiots." And of course, that is something we all wish. But then again, if we didn't have to assume user idiocy, we wouldn't be writing programs.
There's a fundamental difference between constructing a program and writing a novel or painting a picture. For static forms of media, you only have to create one thing from beginning to end. Once you've finished writing your book, it's over. For better or for worse, your characters have finished interacting with each other. They've said what they have to say. People will either like it or not like it; they may debate endlessly about what your words or images "really mean," but they can't affect its behavior.
Unfortunately, programs aren't like that. Once your release your program into the wild, users get to do whatever they want with it. And if they do something utterly crazy and your program breaks, they'll blame you.
Murphy's Law ("Whatever can go wrong, will") was written by an engineer in the 19th century, but it is used most by software engineers. It's not that everything possible will go wrong every single time your program is run. It's just that if you have millions of users (which you will if you are successful) then even a very small chance that one person will do something unexpected, must necessarily magnify into a virtual certainty that somebody, somewhere will find a way to blow up your program.
With that in mind, writing a program is just as much about covering every possible angle of what some idiot user might do to you, as it is about creating a pleasing presentation when the program is Used As Intended.
As a gamer, I have heard several interviews with voice actors who are veterans of film or television, but new to performing voices for video games. The universal sentiment seems to be "This is the craziest thing I've ever had to do. You have to perform a dozen different lines for every single scene, and they're not just different takes for the editor to select from. They're ALL USED in the game. I'll have to perform a scene featuring my dramatic death, and in the very next scene I'm alive again. I'll have to answer the same question five different ways, just in case the player decides to harass me by asking my character over and over again." And so on.
So, because we must cover every possible use of the program, we create elaborate error scenarios and use all kinds of tricks to keep the user on track. One way to handle user error is to simply give the user very strict instructions, like this: "You MUST TYPE A NUMBER from 1-100, and if you do ANYTHING ELSE, then the program WILL CRASH and that is YOUR RESPONSIBILITY." That's not very satisfying, though. People make mistakes, even people who are not idiots. It's much nicer to recover gracefully from an error. At every step, you're asking yourself: "What can go wrong here?" And then you add a clause to your program: "If bad thing xyz occurs, say something polite to guide the user back on track, and try it again."
Often, an even better solution is to tie the user's hands so he can't actually make the mistake in the first place. For example, slider bars and drop boxes exist exactly so that the user can pick an integer from 1-100 without the capability to do something stupid.
Obviously, it takes a lot of work to write a bulletproof program, and the more thoroughly you prepare for bad user behavior, the harder the work is going to be. Often you have to strike a happy medium. The smaller your audience is, the less effort you have to put into mistrusting your users. If you are just writing a program for yourself, it's probably less work to try and give good input than to write error handling routines.
Also, the more general the application, the more you have to just let the errors happen, and your only responsibility becomes to make sure the errors don't cause a crash. For instance, if you're writing a calculator program, you can't just stop all operations that divide by zero. The user might just DECIDE to divide a number by zero. You just have to tell him he made an illegal operation, and move on to the next step.
When you write a program, you're not designing a static thing for people to look at. You're designing a universe of possibilities to cover all uses. The more time you spend thinking about what an idiot might do, the better you can guarantee that you will make it a pleasant experience for those who are not idiots.