How to choose value for unusual digital input signals
In logic design we usually define such an "illegal" combination as a "don't care" combinations, meaning that the implementation might choose any behavior (output) when such an input is seen. Such an approach allows to come up with most optimal design for the "legal" inputs.
 That's fine if we assume the "illegal" input won't happen, or if it will happen but there is nothing bad could be caused by the  possible behavior/output of the system. But if we know that the illegal combination can happen with a non-negligible probability and a possible output can cause some harm, or just unwanted - it means that this combination is not really a "don't care" but a combination that has to be taken in account and a specific behavior has to be defined for it. Which is effectively making it into an "legal" input with a specific output.