What is the general consensus on user-error correction for web apps?

Question

I'm building a RoR site, and today I get the pagination done. Upon showing it to my coworker, his first question is "what happens if you set the querystring to "?page=-1". It died with a runtime exception (error 500). He suggested that that should definitely be fixed before this site goes anywhere near live.

I happen to disagree with him (hear me out). Now, I've been in the web dev business for all of four months, so I very well could be wrong. But I would think that this isn't a big deal. I would think that, so long as said errors do not constitute a security risk, things like this shouldn't be a priority. The only way to cause this error is if you manually edit the query string, and, well, garbage in garbage out. If you're smart enough to know that you even can edit the querystring, you should be smart enough to not give it a negative number.

What is the general consensus on things like this? Do you completely idiot proof the site, so that no matter what the query string is, you never generate an error? Do you let things slide so long as it works the way it's supposed to (and doesn't expose a security risk)? Somewhere in the middle?

EDIT: Somehow my question didn't really come out completely as I intended it. The crux of my question was, where to draw the line between proactively correcting for things versus not doing them. If there's invalid input in the get string, for instance, would it be better practice to display a tasteful error as suggested in the posted replies, or to try to figure out what the user was doing, and do that. Or, as a more concrete example: If a user sets page=-1 in the get string, would it be better to silently assume they meant page=0, or to display some kind of tasteful error page saying somethign like "invalid page specified"?

Answer 1

You should be error checking anything that comes in from the query string. If you get an invalid page number, you should have an error message that's a little more graceful than the Error 500 page. Maybe a sorry, bad request. Try this: <possible suggestions>. It's just plain sloppy and unprofessional to knowingly and deliberately leave an easily accessible error like that on a live site.

You say you're new to web apps, but if your previous dev experience was other GUI apps being used by the "general public" (non-developers, non-techies), would it have been OK to have stack traces thrown into the user's face as the app falls apart around them? In my experience, this is never really acceptable.

Answer 2

You make some good points, but an incorrect query string can have many reasons. For example, a link to a record that has since been deleted. Or a Google result pointing to a page that doesn't exist in the current result set any more.

In these cases, you should show the user something a bit more verbose than a 500 error.

Answer 3

I don't think a 500 error page is very meaningful to your average user. At least tell him something is wrong with your page and guide him back on the right track by providing a link to get back to your site.

Sometimes I redirect users to a page that is likely to what he wanted. So when a query goes below zero and this is not permitted, redirect your user to ?page=0 and maybe display a message on top of that page. I think you should prefer this method because it is a better approach in terms of user experience to not use modal windows.

Answer 4

If you have an error-page that looks nice, and gives a polite message, I'd say it's fine. Though I might consider responding with a 404 instead. Garbage in should preferably not produce an error.