The Modal Fallacy / Modal Scope Fallacy occurs when the modal operator of necessity is applied to the consequence instead of the conditional. This fallacy is a confusion of the necessitas consequentiae with the necessitas consequentiis, the necessity of the consequence with the necessity of the consequent.
“Necessarily, whatever happens, happens. Therefore, whatever happens, necessarily happens, so that whatever occurs could not have been otherwise.”
“If John is a bachelor, then he must be single. If John is single, he cannot be married. Therefore, John can never marry.”