So self checkers never give false negatives, you're saying. The box can't be broken in such a way that it appears to be readable but is giving the wrong sum?
EDIT: It's possible that the installer would give a 'wrong sum' if damaged (incredibly incredibly incredibly unlikely), but...
Even if the box/installer was only slightly damaged and gave a different sum, it's pretty much impossible that the damaged sum will match the contents.
The 'wrong' sum would say that game is damaged, which it is, because the installer (which includes the game data) is damaged to give the wrong sum. It's pretty much fail-proof.