Like everything these days, great care must be taken to ensure that as few people as possible are offended by anything. Google Instant is no exception. Somewhere within Google there exists a master list of "bad words" and evil concepts that Google Instant is programmed to not act upon, lest someone see something offensive in the instant results... even if that's exactly what they typed into the search bar. Its the Google Blacklist.
Go visit Google Blacklist for the list!