Program Repair Shop

"Can't happen"

But there are other kinds of "can't happen":

        if ($foo > 0) { ... }
        elsif ($foo <= 0) { ... }

Where's the else clause?
"Oh, that can't happen"
And this time the author is right
- Unless there is a bug in the implementation or in the universe
Both are outside the scope of the project
Otherwise you are going down the path to madness:

        # Accumulate total payroll expenses

        # But first, make sure Perl's addition operator works correctly
        for (1..100) { $total += $_ }
        die "Addition failure" unless $total == 5050;

        # TODO: this assumes that the == operator works correctly
        #   add more tests!