Comment by TeMPOraL

Nice sentiment, but exactly nothing outside of purely theoretical mathematical constructs work like this. Hell, even math doesn't really work like this, because people occasionally make mistakes in proofs.

EDIT: think of it this way: you may create a program that clearly makes it impossible for a variable X to be 0, and you may even formally prove this property. You may think this mean X will never be 0, but you'd better not wager anything really important over it, because no matter what your proof says, I can still make X be 0, - and I can do it with just a banana. Specifically, by finding where in memory X is physically being stored, and then using the natural radioactivity of a banana to overwrite it bit by bit.

Now imagine X=0 being the impossible stock market crash. Even if you formally prove it can't happen, as long as it's a meaningful concept, a possible state, it can be reached by means other than your proven program.