Apparently, you never know what people will want to put on a vanity plate.
There was an article
on nj.com in February 2013 which features some bad vanity attempts.
Attached to that article was a PDF document from November 2012
with some more banned combinations.
This isn't by any means all of them (I could think of several that should be on the list ;-) ... but it's a start.
Below are some photoshopped graphics of banned plates (taken from the article).
And, strangely enough, these were "allowed".