Not accommodating all the "what if?" s can be the downfall of software.
I was nearly ready to submit my income tax filing when I discovered that the most-commonly-used preparation software had ignored one of the sub-forms. It took me a long time to find that I had made the error of calling it sub-form "1" (one) rather than "01" (zero one). The software wasn't smart enough to handle this. I haven't used that brand of software since then.