You could see it coming. When states and provinces discovered they could make extra bucks by selling "vanity plates" -- personalized licence plates with your choice of lettering -- they quickly compiled lists of verba prohibita. Included were obvious ones, like "SHIT", hence the sayings "You can't write 'SHIT'" and "You don't know 'SHIT'".
Finding the offence in some was great exercise for the Puritanical types so often found in government offices. Plates that somehow got by the censors were recalled following tips or complaints from the blue-stocking brigade. Examples from Ontario include "ANY001", said to have been registered to one of Toronto's top hookers, and "WOMEN". Why "WOMEN"? Because, when the whole text, including the name of the province and an insipid advertising slogan were taken together, they read "ONTARIO WOMEN YOURS TO DISCOVER". Shame!
The problem is that your average North American motorist is cleverer than the by-the-book squares in the DMVs. The snivel servants need crystal balls to go along with the brass ones of some of the people who apply for seemingly-but-not-really innocent tags. Trying to beat the system is just human nature, after all. Here are a couple that got issued only to be yanked back.
It doesn't seem to matter which state or province you're in. If the PC police can find anything -- anything at all -- "morally objectionable" in your chosen combination of letters and/or numbers, they'll turn you down. California has expressively stated it will not issue any plates with "69" in them unless that is the year of the car. And in Manitoba, a Star Trek fan lost his plate that read "ASIMIL8". He'd had it two years, mounted in a plate cover with Star Trek slogans, but the province finally deemed it insensitive and offensive to indigenous people.
Most jurisdictions run applications through a gamut of meanings tests, and also consider the combination being read upside down or in a mirror. In most cases they stay ahead, but the Manitoba plate triggered cries about free speech. They also do Google searches and hit translator sites, much as car manufacturers do when naming a car. If the request means something rude in Ukrainian, it'll be stubbed out. Or so they say, but Agent 5 [Are you still out there, buddy? Ed.] got one for his father-in-law that read "XIMHO". (Send SASE for translation.)
Nowadays they need sites like Urban Dictionary to stay current. The online reference site moves as fast as our changing culture, often setting it. If Urban Dictionary says it's rude, your chances just tumbled. Of course, what might be okay today might well enter the alt-vernacular tomorrow, and your once-innocent plate could still be called back. I'm going to see if "ALT-RT" is still available....