Hint: mononyms are one example, but there are other real valid names besides “Null” that are also not accepted in many places. Surnames of three characters or less also cause problems in broken-by-design computer systems.

Not going to dox myself, but for many years I had a driver’s license and other “official” documentation that showed my name as something like “John Computer-Error-ABCXYZ-Is-A-Valid-Surname”. Then it cascades - when you go to open a bank account, they will only use the “official” surname on your driver’s license or government ID, so they quite happily enter “Computer-Error-ABCXYZ-Is-A-Valid-Surname” into the next computer system.

I really didn’t care - I don’t work for the bank or the state government, so their computer problems are their problem, not mine. I thought it was hilarious to have a driver’s license in the name of “John Computer-Error-ABCXYZ-Is-A-Valid-Surname” and correspondence sent to “Dear Mr Computer-Error-ABCXYZ-Is-A-Valid-Surname”.

A series of news stories on TV and newspapers over several years finally shamed local bureaucracies into fixing their systems.

See also https://www.usatoday.com/story/news/nation/2013/12/30/hawaii-long-name/4256063/ for example