You know how old programmers keep saying nasty sounding things like: "Never trust user input?". This is why.

They don't mean that users are inherently untrustworthy, just that they'll forget what rules they agreed to abide by, and then the software won't work, and they'll call you in confusion, and then their problem will become your problem.

Make your software verify any constraints on user inputs before you assume the input is valid. Suppose the user should enter "T" for true, or "F" for false. Your code should handle the case where someone decides, against all odds, to type "Jello" in that field instead. Because one day, it *will* happen.

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Ytrew