@benjamine although no error is shown, it isn't redefined. The same happens if you try to redefine window.document, for example.
For ie8, you're right, it is possible to redefine undefined, however, if your code runs in strict mode in modern browsers, it won't be a problem in ie8.
well if you want to support ie8 (please don't!! :)), if your code works fine on modern browsers with strict mode, you can still have errors on ie8, even worse, you can get errors that only show up on ie8, so testing gets harder.
Of course though if someone redefines undefined should go to prison for the rest of his/er life :)