Definition:Can't Happen/Computer Science

Definition
A branch of a program which, because of the way the flow is structured, cannot be accessed, is referred to as can't happen:

if (x == 1) then { do.something (x) } else if (x != 1) then { do.somethingElse (x) } else { // can't happen }

There are sometimes reasons for doing stuff like this, but professionals usually wince when they have to maintain it.