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 :-/