Actually neither is a mistake at all--in English, a double negative
just means something different from what was intended, but is valid
grammatically. Putting a preposition at the end of a sentence is
also a perfectly valid practice in English, and any teacher who tells
you otherwise should be summarily fired for incompetence.
But let's not start a grammar war here (though I'd be happy to
start one in private email :)
-- Lee Daniel Crocker <lee@piclab.com> <http://www.piclab.com/lcrocker.html> "All inventions or works of authorship original to me, herein and past, are placed irrevocably in the public domain, and may be used or modified for any purpose, without permission, attribution, or notification."--LDC