"Not" is a very important word - leave the "not" out of any sentence, and you end up with a very different meaning. However, library catalogs don't necessarily care about that kind of thing, which can create a bit of a disconnect between them and their users.
Take this book I just cataloged: I can't believe it's not fattening! : 0ver 150 ridiculously easy recipes for the super busy by Devin Alexander. You won't ever find this book by searching for I can't believe it's not fattening. You'll either have to search for it with everything in quotes, or (the more amusing option) you'll have to search for I can't believe it's fattening, leaving out the "not."
I've heard that there's maybe one ILS out there that doesn't ask its users to rely on special tricks for getting around the "special Boolean words" issue. Ours is not that ILS.