Sometimes it's the sanity checks themselves which fail to handle input that the system itself could handle fine.
Do you know that "the system" can handle that input fine? If you do, why did you sanity check reject it?
Sanity checks are just code -- you certainly can write bad ones. So?
You don't want to end up running into the Y2Gay problem
That post argues via mind-numbingly stupid strawmen (or should that be strawschemas?). Yes, you should try to be not stupid, most of the time, to the best of your ability. I agree :-/
There are some long lists of false beliefs that programmers hold. isn't because programmers are especially likely to be more wrong than anyone else, it's just that programming offers a better opportunity than most people get to find out how incomplete their model of the world is.
I'm posting about this here, not just because this information has a decent chance of being both entertaining and useful, but because LWers try to figure things out from relatively simple principles-- who knows what simplifying assumptions might be tripping us up?
The classic (and I think the first) was about names. There have been a few more lists created since then.
Time. And time zones. Crowd-sourced time errors.
Addresses. Possibly more about addresses. I haven't compared the lists.
Gender. This is so short I assume it's seriously incomplete.
Networks. Weirdly, there is no list of falsehoods programmers believe about html (or at least a fast search didn't turn anything up). Don't trust the words in the url.
Distributed computing Build systems.
Poem about character conversion.
I got started on the subject because of this about testing your code, which was posted by Andrew Ducker.