Definition:Can't Happen

Probability Theory
Let $A$ be an event in a probability space $\left({\Omega, \Sigma, \Pr}\right)$.

Suppose $\Pr \left({A}\right) = 0$.

Then we say (colloquially) that $A$ can't happen.

Computer Science
A branch of a program which, because of the way the flow is structured, can not be accessed, is referred to as a 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.