For a mathematician, the line $$x = x + 1$$is an equation with no solution: subtracting $x$ from both sides leaves $0 = 1$. For a programmer, it is one of the most common lines there is, and it means “take the value of $x$, add one and store the result in $x$”. The same sign, in the two communities, does opposite jobs: there it asserts an equality; here it orders an assignment.