Modal fallacy

The formal fallacy of the modal fallacy is a special type of fallacy that occurs in modal logic. It is the fallacy of placing a proposition in the wrong modal scope, specifically inferring that because something is true, it is necessarily true.

In modal logic, a proposition ${\displaystyle P}$ can be necessarily true or false (denoted ${\displaystyle \Box P}$ and ${\displaystyle \Box \lnot P}$, respectively), meaning that it is logically necessary that it is true or false; or it could be possibly true or false (denoted ${\displaystyle \diamond P}$ and ${\displaystyle \diamond \lnot P}$), meaning that it is true or false, but it is not logically necessary that it is so: its truth or falseness is contingent. The modal fallacy occurs when there is a confusion of the distinction between the two.

...
Wikipedia