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".